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