Add pep425 tagging support to twitter.common.python

This adds PEP425 tag iteration to twitter.common.python:
http://legacy.python.org/dev/peps/pep-0425/

Wheel platform tags are encoded in this fashion and is the standardized way
to determine whether or not a wheel distribution is compatible with the
current platform.

Reviewed at https://rbcommons.com/s/twitter/r/87/