Move test requirements into another file so they might be referenced manually.

tests/requirements.txt

+pytest-flake8
+pytest>=3.0.2
+setuptools[ssl]
+backports.unittest_mock>=1.2

tox.ini

 [testenv]
-deps=
-	pytest-flake8
-	pytest>=3.0.2
-	setuptools[ssl]
-	backports.unittest_mock>=1.2
-
+deps=-rtests/requirements.txt
 passenv=APPDATA USERPROFILE HOMEDRIVE HOMEPATH windir
 commands=python -m pytest {posargs:-rsx}