Removes and/or replaces all mentions of py27.
Cleans up obsolete requirements and their lower-constraints.
Separates test-requirements.
Makes lower-constraints pass outside of CI (MarkupSafe).
Adds FIXMEs about some hacky Mocks that may misbehave.
Change-Id: Ifc090bf3c1db17d8542ee591c91e8225a597bfe2
234272eb64