Avoid use of sys.version for checking Python version

[svisser]

This changes the Python 2 / 3 version check to use sys.version_info as the version number is not guaranteed to be at the beginning of sys.version across Python implementations. Please also see the note in the documentation: https://docs.python.org/2/library/sys.html#sys.version.

[svisser]

The build fails but that's due to no such option: --use-mirrors when calling pip, not this code change.