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.
Signed-off-by: Doug Hellmann <email@example.com>
According to Openstack summit session ,
stestr is maintained project to which all Openstack projects should migrate.
Let's switch to stestr as other projects have already moved to it.