Maybe MYPY, PY2 and PY3 can be imported from typing instead of having to set them?

[gvanrossum]

I have to write code like this:

MYPY = False
if MYPY:
   # do mypy stuff

I think it would be more elegant if I could write

from typing import MYPY

instead. That way the values can actually be defined properly in the stubs and lib-typing files.

[JukkaL]

Yeah, that's a good idea.

[JukkaL]

MYPY doesn't make sense any more in the context of PEP 484.

[JukkaL]

typing.PY2 and typing.PY3 were removed from PEP 484. We should detect runtime Python version checks instead. Created #698 to supercede this issue.