tox: Update six to 1.10.0.

Upstream changelog:

- Improve the performance of `six.int2byte` on Python 3.
- Don't add the `winreg` module to `six.moves` on non-Windows platforms.
- Add `six.moves.getcwd` and `six.moves.getcwdu`.
- Add `create_unbound_method` to create unbound methods.

tox.ini

@@ -90,7 +90,7 @@ deps =
     astroid==1.3.8
     pylint==1.4.4
     logilab-common==1.0.2
-    six==1.9.0
+    six==1.10.0
     vulture==0.8.1
 commands =
     {envpython} scripts/link_pyqt.py --tox {envdir}