It should be possible to type check code that uses conditional Python version checks.

Consider this code:

if PY3:
    # python 3
    ...
else:
    # python 2
    ...

If type checking in Python 3 mode, we should only type check the if block and ignore the else block, and vice versa in Python 2 mode.

Open questions:

• How do we determine which flag/constant the code uses to check for Python version?
• Should we also have special support for six?