Instead of running a whole additional test matrix for constraints /
non-constraints, we should consider tox -e py27 the interface for
running unit tests with python2.7, and exactly which dependencies are
installed (a range with *-requirements, or an exact version with
upper-constraints) is an implementation detail which can be opted into
by each project if they feel it is appropriate.
Change-Id: I118831c5e7cdb5de9ae23387b6af92869cca6827
3687003d13