Implement most common sys.version_info and sys.platform checks

[gvanrossum]

Addresses most but not all of #698. (The first two bullets of my plan.)

[JukkaL]

Also this won't work with things like import sys as _sys -- I found 8 files that did this in the Python 2.7 stdlib.

[JukkaL]

Another thing that doesn't work is from sys import platform (which is arguably questionable style and doesn't seem very common). Not sure if we should support it -- one option is to describe it as unsupported in our docs.

[JukkaL]

I did a very quick pass and looks reasonable -- I won't have time to review this in detail since I'm going to be on vacation, so please don't wait until you get more feedback :-)

A few more ideas for test cases (though at least some of these are probably covered by some existing tests):

• A test that has a check within a function or class body instead of module top level.
• A test that conditionally defines a class.
• A test that conditionally imports a module.
• A test that conditionally defines a variable.

[gvanrossum]

Thanks for the review! I've added some TODOs and more tests. I'll merge this once Travis-CI is green.

[eric-wieser]

Supporting sys.version_info.major would be a nice addition

[gvanrossum]

I opened #4661 for this -- but note that we'd need buy-in from PyCharm and Google's pytype as well before we can start using this in stubs (where it's most useful).