tox: Update pytest-mock to 0.4.3.

Upstream changelog:

- mocker and the backward compatible mock fixture now return the same object.

tox.ini

@@ -21,7 +21,7 @@ deps =
     pytest==2.7.0
     pytest-capturelog==0.7
     pytest-qt==1.3.0
-    pytest-mock==0.4.2
+    pytest-mock==0.4.3
 # We don't use {[testenv:mkvenv]commands} here because that seems to be broken
 # on Ubuntu Trusty.
 commands =