sphinx-feature-classification was declared python-3-only by change
I0f3e9d980c25545a in release 1.0.1 (2020-04-07 10:02:33 +0000).
It's never had six in requirements.txt despite importing six in
two files. Now that consuming projects are all python-3-only,
six is not being pulled in by some other project's requirements.
So convert six usage to pure python 3.
Change-Id: I1917fda331860332242774019ac7ca54512430e3
f8909fc1d5