Dialect

[o11c]

I've pulled this out of #880 because it promises to be useful for #884 and #732

[o11c]

On further thought, I should separate out mypy-codec, mypy-stub, and add mypy-string for lexing the interior of string type annotations.

[o11c]

Expected usage:

• From the command-line tool:
  • Add a --python-executable option (do this after driver so that it's all in mypy.main instead of scripts/mypy), store its argument in a python_executable variable.
  • set force_py2 = True if the --py2 flag is passed, for backward compatibility, but deprecate --py2.
  • python_implementation = dialect.Implementation(python_executable) if python_executable is not None else dialect.default_implementation(force_py2=force_py2)
  • Deprecate the --use-python-path option since we can now use it correctly even in python2 mode.
• From the testsuite, see Improve test coverage of the Python2 bits of mypy #732, which I am increasingly convinced needs to be done before syntax, for testsuite reasons.