tox: Update pytest-mock to 0.10.0.

pytest-mock now monkeypatches the mock library to improve pytest output
for failures of mock call assertions like Mock.assert_called_with().

tox.ini

@@ -32,7 +32,7 @@ deps =
     pytest-cov==2.2.0
     pytest-faulthandler==1.3.0
     pytest-html==1.7
-    pytest-mock==0.9.0
+    pytest-mock==0.10.0
     pytest-qt==1.11.0
     pytest-instafail==0.3.0
     pytest-travis-fold==1.2.0