Use pytest from fork which fixes assertion rewriting in development mode

Author: nicoddemus

tox.ini

@@ -9,7 +9,8 @@ deps =
     coverage
     pytest27: pytest==2.7.3
     pytest28: pytest==2.8.7
-    pytest30: pytest==3.0.1
+    # temporary until pytest-dev/pytest#1935 is merged
+    pytest30: git+https://github.com/nicoddemus/pytest@assert-rewrite-dev-plugins
 commands = coverage run --append --source=pytest_mock.py -m pytest test_pytest_mock.py