tox: Random cleanups

Change-Id: Ib62f264e817d77ed6cda47daeaaa2994fc5d9f01
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tox.ini

@@ -1,15 +1,15 @@
 [tox]
-minversion = 3.1.0
-envlist = py37,pep8,docs
-ignore_basepython_conflict = true
+minversion = 4.3.0
+envlist = py3,pep8,docs
 
 [testenv]
-basepython = python3
-deps = -r{toxinidir}/test-requirements.txt
-commands = stestr run --slowest {posargs}
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/test-requirements.txt
+commands =
+  stestr run --slowest {posargs}
 
 [testenv:pep8]
-deps=-r{toxinidir}/test-requirements.txt
 commands =
   flake8
 
@@ -17,12 +17,13 @@ commands =
 commands = {posargs}
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
-commands = 
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
+commands =
   sphinx-build -a -E -W -d doc/build/doctrees -b html doc/source doc/build/html
 
 [flake8]
-ignore = E123,H405,W504
 show-source = True
-exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,__init__.py
-
+exclude = .venv,.git,.tox,dist,doc,*lib/python*,*egg,build,__init__.py
+ignore = E123,H405,W504