tox: Encode specific Python versions

Declare specific versions of Python for environments that don't
specifically encode a version. We use the highest version officially
supported, Python 3.8, for most environments except lower-constraints,
which uses the lowest version, Python 3.6. This is necessary for users
on Fedora, where the default Python version is Python 3.9.

This will unfortunately introduce a small amount of busy work whenever
we change supported Python versions, as tox will need to be updated, but
that's a small price to pay for the usability wins that Fedora users
will see.

Change-Id: I240d5b8aa3eb1925b97af57e2644be61bfc106c2
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
Depends-On: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/801773

tox.ini

@@ -7,7 +7,7 @@ envlist = py38,functional,pep8
 ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
+basepython = python3.8
 usedevelop = True
 allowlist_externals =
   bash
@@ -342,6 +342,7 @@ commands =
   bindep test
 
 [testenv:lower-constraints]
+basepython = python3.6
 usedevelop = False
 deps =
   -c{toxinidir}/lower-constraints.txt