There is no official release of pytest-mock which supports python3.6

However master already supports it, so lets use master
until a official release is there.

tox.ini

@@ -19,7 +19,7 @@ deps =
     pytest-remove-stale-bytecode
     pytest-flake8
     pytest-isort
-    pytest-mock
+    git+https://github.com/pytest-dev/pytest-mock@931785ca86113c62baaad1e677f5dc61d69ec39a
 #    pytest-mypy
 
 [testenv:coverage-clean]