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: I5d8872b8470edfaf472757d517450dd0fca94bcb
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = py37,pep8
+envlist = py38,pep8
 skipsdist = True
 ignore_basepython_conflict = True