tox: enforce python3 for all environments and fix sphinx-dev for old tox versions

tox.ini

 [tox]
 envlist=flake8
 
+[testenv]
+basepython = python3
+
 [testenv:sphinx]
 deps =
   django < 2
@@ -12,7 +15,7 @@ commands =
 [testenv:sphinx-dev]
 deps =
   django < 2
-  -r requirements-swh-dev.txt
+  -rrequirements-swh-dev.txt
   pifpaf
 commands =
   {envpython} -m pifpaf run postgresql -- make -C docs html