Consider supporting sys.version_info.major (and .minor) #4661

It turns out there's a nicer notation than sys.version_info[0] -- sys.version_info.major. Consider supporting this too, as well as .minor. This is supported by Python 2.7, 3.1 and higher, so all Python versions that mypy supports.

A minor PEP 484 update would be required here: https://www.python.org/dev/peps/pep-0484/#version-and-platform-checking

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).