__requires__ hackery for parallel-installed sphinx versions

This is needed on RHEL6 when both python-sphinx (0.6) and
python-sphinx10 (1.0) are installed at the same time.

Also added an explanatory comment in all the other places where the
hackery already existed for CherryPy.

Change-Id: Ia48327b16634c5a2730764b270dd0a76dd6ab6a1
Merge "__requires__ hackery for parallel-installed sphinx versions" into develop