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.

Even though [1] is fixed we are still seeing this failure.

[1] tox-dev/tox#2811

Also updated basepython to use TOX_PYTHON if set.

Change-Id: I18765db9682fced05e8441a1e2122f9f814cd31b

tox.ini

@@ -1,10 +1,10 @@
 [tox]
 minversion = 3.18.0
-envlist = py38,pep8,docs
+envlist = py3,pep8,docs
 ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
+basepython = {env:TOX_PYTHON:python3}
 usedevelop = True
 passenv = TRACE_FAILONLY
 setenv =