Move mock install to requirements-test

requirements-test.txt

@@ -2,3 +2,4 @@
 pytest==3.6.4
 pytest-mock==1.10.0
 freezegun==0.3.10
+mock==2.0.0 ; python_version<'3.3'

tox.ini

@@ -4,7 +4,6 @@ envlist = flake8,py{27,34,35,36,37},cov-report,cov-store
 [testenv]
 use_develop = true
 deps =
-    py27: mock>=2.0.0
     -r{toxinidir}/requirements-test.txt
     # setup.py has unpinned dependencies by default
     !unpinned: -r{toxinidir}/requirements.txt