The commands used by constraints need at least tox 2.0. Update to
reflect reality, which should help with local running of constraints
targets.
Change-Id: I64ca6750db24cc9a66271062070bbacac08e4f9e
Closes-Bug: #1801479
We want to default to running all tox environments under python 3, so
set the basepython value in each environment.
We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.
We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.
Change-Id: I43b21411d01290ddea7ca5fe11d70e970bf65bb3
Vieri