Bump default tox env from py37 to py38

Python 3.8 is now our highest level supported python runtime.
This updates the default tox target environments to swap out
py37 for py38 to make sure local development testing is
covering this version.

This does not impact zuul jobs in any way, nor prevent local
tests against py37. It just changes the default if none is
explicitly provided.

Change-Id: I4c4c6b3bf20500623035658aa351886f1dad9de5
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py37,pep8
+envlist = py38,pep8
 minversion = 3.1.1
 skipsdist = True
 # this allows tox to infer the base python from the environment name