Adding pytest-mock to tox

tox.ini

@@ -20,6 +20,7 @@ deps =
     py==1.4.26
     pytest==2.7.0
     pytest-qt==1.3.0
+    pytest-mock==0.4.2
 # We don't use {[testenv:mkvenv]commands} here because that seems to be broken
 # on Ubuntu Trusty.
 commands =
@@ -32,6 +33,7 @@ deps =
     coverage==3.7.1
     pytest-cov==1.8.1
     pytest-qt==1.3.0
+    pytest-mock==0.4.2
     cov-core==1.15.0
 commands =
     {[testenv:mkvenv]commands}