add pep8 to tox.ini

Add a separate pep8 environment in tox.ini for linting only the code
changes. We don't need to add the zuul job because flake8 also runs as
part of the linters environment and job.

Change-Id: Ie6bbdfdecef43666cb0eb5d3ff4a34a7067daed2
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -23,6 +23,11 @@ commands =
   {toxinidir}/tools/validate_repositories.py
   {toxinidir}/tools/validate_atcs.py
 
+[testenv:pep8]
+basepython = python3
+commands =
+  flake8
+
 [testenv:venv]
 basepython = python3
 commands = {posargs}