Let -2/--py2 argument destination match --python-version's (#5619)

Fixes #5576. This makes it so that there's no longer a difference between specifying `--python-version=2.7` and using `-2` or `--py2`. (The config file behaves different than any of those, but that's a separate issue.)

Author: sqwishy

mypy/main.py

@@ -465,7 +465,7 @@ def add_invertible_flag(flag: str,
         help='Type check code assuming it will be running on Python x.y',
         dest='special-opts:python_version')
     platform_group.add_argument(
-        '-2', '--py2', dest='python_version', action='store_const',
+        '-2', '--py2', dest='special-opts:python_version', action='store_const',
         const=defaults.PYTHON2_VERSION,
         help="Use Python 2 mode (same as --python-version 2.7)")
     platform_group.add_argument(