bump py37 to py38 in tox.ini

in 'victoria' cycle, we should test py38 by default.

ref:
  https://governance.openstack.org/tc/reference/runtimes/victoria.html

Change-Id: I183db1a6db8ff512a20a3a467770f9b6f1cbd260

tox.ini

 [tox]
 minversion = 3.2.0
-envlist = py37,pep8
+envlist = py38,pep8
 ignore_basepython_conflict = True
 
 [testenv]