tox: Use python3 for all testenvs

Change-Id: I8ec8c65be8448b49ed915262c9026281958e982a
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tox.ini

@@ -1,7 +1,9 @@
 [tox]
 envlist = pep8,docs,py27,py3{4,5,6,7}
+ignore_basepython_conflict = true
 
 [testenv]
+basepython = python3
 # See "testing behind a proxy"
 # @ https://review.openstack.org/#/q/I764ed51314c8e2d0572d1f0
 passenv =
@@ -25,7 +27,6 @@ deps =
 commands = flake8
 
 [testenv:docs]
-basepython = python3
 deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
@@ -37,5 +38,5 @@ commands = {posargs}
 
 [flake8]
 ignore = E125,H202,H405,H904,W504
-show-source = True
+show-source = true
 exclude = .venv,.git,.tox,dist,doc,releasenotes,*lib/python*,*egg,build