tox: extract test dependencies into a test-requirements.txt file

This make testing outside of tox easier.
Also drop "pbr" from dependencies (it was an old workaround to install pifpaf...)

test-requirements.txt

+pytest
+pifpaf
+mock
+redis<3

tox.ini

@@ -3,11 +3,7 @@ envlist = py27,py3,flake8
 
 [testenv]
 deps =
-  pytest
-  pbr>=1.4
-  pifpaf
-  mock
-  redis<3
+  -rtest-requirements.txt
 commands =
   pifpaf run redis --port 6380 -- {envpython} -m pytest {posargs:test}