support platform and version checks in conditional expressions

[benjaminp]

I wanted to write os.environb if sys.version_info[0] == 3 else os.environ to obtain a dictionary of bytes environmental variables. However, this fails to typecheck on Python 2 because mypy doesn't recognize the version in conditional expressions.

[emmatyping]

We do support platform and version checks, but only sys.platform and sys.version_info. Considering six is used a fair amount for Python2/3 straddling code, I think we should also support it.

[benjaminp]

Ah, sorry, I didn't mean this to be about six. There's the same problem with sys.version_info[0] == 3.

[emmatyping]

Ah, interesting, with this:

import sys
import os
reveal_type(os.environb if sys.version_info > (3, 0, 0) else os.environ)

This reveals builtins.object on Python 3 and gives an attribute error on Python 2 (since os.environb doesn't exist). This does indeed seem to be a bug.

[gvanrossum]

(OT: six.PY3 support is easy: just pass --always-true=PY3.)

[ilevkivskyi]

This is a duplicate of #5550.

I will raise the priority of that issue (but still not high probably, since there is a simple workaround).