tox: Update to pytest-qt 1.8.0.

- pytest.mark.qt_log_ignore now supports an extend parameter that will extend
  the list of regexes used to ignore Qt messages (defaults to False).

- Fixed internal error when interacting with other plugins that raise an error,
  hiding the original exception.

tox.ini

@@ -18,7 +18,7 @@ deps =
     py==1.4.30
     pytest==2.7.3  # rq.filter: <2.8.0
     pytest-capturelog==0.7
-    pytest-qt==1.7.0
+    pytest-qt==1.8.0
     pytest-mock==0.8.1
     pytest-html==1.6
     pytest-faulthandler==1.0.1