tox: use requirements files to specify dependencies

Author: benoit-pierre

tox.ini

@@ -4,6 +4,6 @@ skip_missing_interpreters = true
 
 [testenv]
 deps=
-	nose
-	six>=1.10.0
+	-rdev-requirements.txt
+	-rrequirements.txt
 commands={envpython} -W all runtests.py {posargs}