Ned Deily: "I'll be doing backports to 2.7 for this and some other things shortly."

It seems like Ned was busy or forgot 2.7, so I wrote a change: https://github.com/python/cpython/pull/1327

My change doesn't touch platform.py. platform._sys_version() uses sys.subversion but not sys._hg. If someone wants to enhance the platform module of Python 2.7, I suggest to open a new issue. There is a non-zero risk of breaking the backward compatibility, so I skipped my turn for that one :-)