Temporary update envlist for tox4

tox4 even when running specific env with -e option tries
to find python version for all envs defined in envlist[1]
when running with --skip-missing-interpreters=false and
fails if interpreter missing for any of the env.

With py38 in envlist it tries to find python3.8
which is not available in ubuntu-jammy by default
and fails, hence use generic version instead i.e py3
to handle it.

Once [1] is fixed we can update the list as per
default supported py3 version for a release.

[1] tox-dev/tox#2811

Change-Id: I3afc539c59f223338920d7fd59f0df76e842ece1

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = docs,py38,pep8
+envlist = docs,py3,pep8
 minversion = 3.18.0
 skipsdist = False
 ignore_basepython_conflict = True