tox: Stop using 'build_sphinx'

Plus some other modern tox niceties.

Change-Id: I7a3ed2c1aa039af9ae59f35d74365a7f1d626037

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.1.1
 envlist = pep8
 
 [testenv]
@@ -10,18 +10,16 @@ deps =
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 
-[testenv:py27]
-basepython = python2.7
-
 [testenv:pep8]
 commands =
-    flake8 oslosphinx
+  flake8 oslosphinx
 
 [testenv:venv]
 commands = {posargs}
 
 [testenv:docs]
-commands = python setup.py build_sphinx
+commands =
+  sphinx-build -a -E -W -d doc/build/doctrees -b html doc/source doc/build/html
 
 [flake8]
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,tools