tox: Upgrade CherryPy to 3.8.0.

Pass ``exc_info`` to logger as keyword rather than formatting the error and
injecting into the message.

tox.ini

@@ -42,7 +42,7 @@ deps =
     Werkzeug==0.11.2
     wheel==0.26.0
     xvfbwrapper==0.2.5
-    cherrypy==3.8.0
+    cherrypy==3.8.1
 commands =
     {envpython} scripts/link_pyqt.py --tox {envdir}
     {envpython} -m py.test --strict -rfEsw --faulthandler-timeout=70 --cov --cov-report xml --cov-report=html --cov-report= {posargs:tests}