tox: Update mccabe to 0.3.1.

Upstream changelog:

- Include test_mccabe.py in releases.
- Always coerce the max_complexity value from Flake8's entry-point to an integer.

tox.ini

@@ -102,7 +102,7 @@ deps =
     -r{toxinidir}/requirements.txt
     py==1.4.28
     pytest==2.7.1
-    mccabe==0.3
+    mccabe==0.3.1
     pytest-mccabe==0.1
 commands =
     {envpython} scripts/link_pyqt.py --tox {envdir}