Revert "Update -2 argument dest to match --python-version."

[ilevkivskyi]

Reverts #5578 @ethanhs @sqwishy

It looks like the original PR caused breakages. We can re-apply later when we will have a fix.

In particular PEP 561 packages are now not available in virtual environments, for example three of mypy tests fail when running then in a venv:

_______________________________________________________________ TestPEP561.test_typedpkg_python2 ________________________________________________________________
[gw7] darwin -- Python 3.6.6 /Users/ilevkivskyi/mypy_dev/bin/python

self = <mypy.test.testpep561.TestPEP561 testMethod=test_typedpkg_python2>

    def test_typedpkg_python2(self) -> None:
        python2 = try_find_python2_interpreter()
        if python2:
            with self.virtualenv(python2) as venv:
                venv_dir, py2 = venv
                self.install_package('typedpkg', py2)
                check_mypy_run(
                    [self.tempfile],
                    py2,
                    expected_out=self.msg_tuple,
>                   venv_dir=venv_dir,
                )

mypy/test/testpep561.py:170: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cmd_line = ['/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpc0rr57i4/simple.py', '--python-executable=/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmp_5okw9zi/bin/python']
python_executable = '/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmp_5okw9zi/bin/python'
expected_out = "/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpc0rr57i4/simple.py:5: error: Revealed type is 'builtins.tuple[builtins.str]'\n"
expected_err = '', expected_returncode = 1, venv_dir = '/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmp_5okw9zi'

    def check_mypy_run(cmd_line: List[str],
                       python_executable: str = sys.executable,
                       expected_out: str = '',
                       expected_err: str = '',
                       expected_returncode: int = 1,
                       venv_dir: Optional[str] = None) -> None:
        """Helper to run mypy and check the output."""
        if venv_dir is not None:
            old_dir = os.getcwd()
            os.chdir(venv_dir)
        try:
            if python_executable != sys.executable:
                cmd_line.append('--python-executable={}'.format(python_executable))
            out, err, returncode = mypy.api.run(cmd_line)
>           assert out == expected_out, err
E           AssertionError: usage: mypy [-h] [-v] [-V] [more options; see below]
E                         [-m MODULE] [-p PACKAGE] [-c PROGRAM_TEXT] [files ...]
E             mypy: error: Python version (3, 6) did not match executable /var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmp_5okw9zi/bin/python, got version (2, 7).
E             
E           assert '' == "/var/folders/ff/8tg5kw8x3jg...tins.tuple[builtins.str]'\n"
E             + /var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpc0rr57i4/simple.py:5: error: Revealed type is 'builtins.tuple[builtins.str]'

mypy/test/testpep561.py:39: AssertionError
--------------------------------------------------------------------- Captured stderr call ----------------------------------------------------------------------
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AssertionError
_______________________________________________________________ TestPEP561.test_typedpkg_editable _______________________________________________________________
[gw0] darwin -- Python 3.6.6 /Users/ilevkivskyi/mypy_dev/bin/python

self = <mypy.test.testpep561.TestPEP561 testMethod=test_typedpkg_editable>

    def test_typedpkg_editable(self) -> None:
        with self.virtualenv() as venv:
            venv_dir, python_executable = venv
            self.install_package('typedpkg', python_executable, editable=True)
            check_mypy_run(
                [self.tempfile],
                python_executable,
                expected_out=self.msg_tuple,
>               venv_dir=venv_dir,
            )

mypy/test/testpep561.py:192: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cmd_line = ['/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpt1nk7wi5/simple.py', '--python-executable=/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpte9h7lad/bin/python']
python_executable = '/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpte9h7lad/bin/python'
expected_out = "/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpt1nk7wi5/simple.py:5: error: Revealed type is 'builtins.tuple[builtins.str]'\n"
expected_err = '', expected_returncode = 1, venv_dir = '/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpte9h7lad'

    def check_mypy_run(cmd_line: List[str],
                       python_executable: str = sys.executable,
                       expected_out: str = '',
                       expected_err: str = '',
                       expected_returncode: int = 1,
                       venv_dir: Optional[str] = None) -> None:
        """Helper to run mypy and check the output."""
        if venv_dir is not None:
            old_dir = os.getcwd()
            os.chdir(venv_dir)
        try:
            if python_executable != sys.executable:
                cmd_line.append('--python-executable={}'.format(python_executable))
            out, err, returncode = mypy.api.run(cmd_line)
>           assert out == expected_out, err
E           AssertionError: 
E           assert "/var/folders...pe is 'Any'\n" == "/var/folders/...ltins.str]'\n"
E             Skipping 61 identical leading characters in diff, use -v to show
E             + simple.py:5: error: Revealed type is 'builtins.tuple[builtins.str]'
E             - simple.py:2: error: Cannot find module named 'typedpkg.sample'
E             - /var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpt1nk7wi5/simple.py:2: note: (Perhaps setting MYPYPATH or using the "--ignore-missing-imports" flag would help)
E             - /var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpt1nk7wi5/simple.py:3: error: Cannot find module named 'typedpkg'
E             - /var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpt1nk7wi5/simple.py:5: error: Revealed type...
E             
E             ...Full output truncated (1 line hidden), use '-vv' to show

mypy/test/testpep561.py:39: AssertionError
____________________________________________________________ TestPEP561.test_typedpkg_stubs_python2 _____________________________________________________________
[gw7] darwin -- Python 3.6.6 /Users/ilevkivskyi/mypy_dev/bin/python

self = <mypy.test.testpep561.TestPEP561 testMethod=test_typedpkg_stubs_python2>

    def test_typedpkg_stubs_python2(self) -> None:
        python2 = try_find_python2_interpreter()
        if python2:
            with self.virtualenv(python2) as venv:
                venv_dir, py2 = venv
                self.install_package('typedpkg-stubs', py2)
                check_mypy_run(
                    [self.tempfile],
                    py2,
                    expected_out=self.msg_dne + self.msg_list,
>                   venv_dir=venv_dir,
                )

mypy/test/testpep561.py:157: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cmd_line = ['/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpdsnd7z59/simple.py', '--python-executable=/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpgjk1xd6p/bin/python']
python_executable = '/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpgjk1xd6p/bin/python'
expected_out = "/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpdsnd7z59/simple.py:3: error: Module 'typedpkg' has no attribute '...s/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpdsnd7z59/simple.py:5: error: Revealed type is 'builtins.list[builtins.str]'\n"
expected_err = '', expected_returncode = 1, venv_dir = '/var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpgjk1xd6p'

    def check_mypy_run(cmd_line: List[str],
                       python_executable: str = sys.executable,
                       expected_out: str = '',
                       expected_err: str = '',
                       expected_returncode: int = 1,
                       venv_dir: Optional[str] = None) -> None:
        """Helper to run mypy and check the output."""
        if venv_dir is not None:
            old_dir = os.getcwd()
            os.chdir(venv_dir)
        try:
            if python_executable != sys.executable:
                cmd_line.append('--python-executable={}'.format(python_executable))
            out, err, returncode = mypy.api.run(cmd_line)
>           assert out == expected_out, err
E           AssertionError: usage: mypy [-h] [-v] [-V] [more options; see below]
E                         [-m MODULE] [-p PACKAGE] [-c PROGRAM_TEXT] [files ...]
E             mypy: error: Python version (3, 6) did not match executable /var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpgjk1xd6p/bin/python, got version (2, 7).
E             
E           assert '' == "/var/folders/ff/8tg5kw8x3jg...ltins.list[builtins.str]'\n"
E             + /var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpdsnd7z59/simple.py:3: error: Module 'typedpkg' has no attribute 'dne'
E             + /var/folders/ff/8tg5kw8x3jg3z9cw0359hllxss39wp/T/tmpdsnd7z59/simple.py:5: error: Revealed type is 'builtins.list[builtins.str]'

mypy/test/testpep561.py:39: AssertionError