update pep8 test in tox

Change-Id: I4cbd530b1a3226361b298e9b0886658dae088c0b

tox.ini

@@ -19,9 +19,13 @@ deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/up
 commands = stestr run {posargs}
 
 [testenv:pep8]
+deps =
+  -r{toxinidir}/doc/requirements.txt
+  -r{toxinidir}/test-requirements.txt
 commands =
-    flake8 {posargs}
-    doc8 specs/
+  find . -type f -name "*.pyc" -delete
+  stestr run --slowest {posargs}
+  flake8 {posargs}
 
 [testenv:venv]
 commands = {posargs}