Update tox configuration to match RTD docs build

Keep the build type as html, though, for easier local navigation.

Author: samdoran

tox.ini

@@ -27,5 +27,6 @@ commands=
 [testenv:docs]
 description = Build documentation
 deps = -r{toxinidir}/docs/requirements.txt
+skip_install = True
 commands =
-    sphinx-build -E -W -d docs/build/doctrees -b html docs docs/build/html
+    sphinx-build -T -E -W --keep-going {tty:--color} -j auto -d docs/build/doctrees -b html docs docs/build/html