Use py3 as the default runtime for tox

Moving on py3 as the default runtime for tox to avoid to update this at
each new cycle.

Change-Id: I9e3eb3aca01fb94a92153e26659c2c412de98093

tox.ini

 [tox]
 minversion = 3.18
-envlist = py38,linters
+envlist = py3,linters
 ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
 usedevelop = True
 allowlist_externals = bash
-                      find 
+                      find
                       rm
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt