Add a py3pep8 tox job. This will verify py3 compliant syntax

At this time barbican has several syntax errors that prevent this from
becoming a voting gate

Change-Id: I652f561264a7b389af23ee7b3569f893dfbc021e

tox.ini

@@ -24,6 +24,15 @@ commands = flake8 {posargs}
 [testenv:venv]
 commands = {posargs}
 
+[testenv:py3pep8]
+# This hack is in place to allow us to run py3 based flake8
+# without installing barbican.
+basepython = python3
+install_command = /bin/echo {packages}
+commands =
+  pip install "hacking>=0.8.0,<0.9"
+  flake8 barbican setup.py
+
 [testenv:docs]
 envdir = {toxworkdir}/venv
 commands=