fixup! tox.ini: clean up filterwarnings

Author: blueyed

tox.ini

@@ -133,6 +133,8 @@ filterwarnings =
     ignore::pytest.RemovedInPytest4Warning
     default:Using or importing the ABCs:DeprecationWarning:unittest2.*
     ignore:Module already imported so cannot be rewritten:pytest.PytestWarning
+    # produced by python3.6/site.py itself (3.6.7 on Travis, could not trigger it with 3.6.8).
+    ignore:.*U.*mode is deprecated:DeprecationWarning:(?!(pytest|_pytest))
     # produced by pytest-xdist
     ignore:.*type argument to addoption.*:DeprecationWarning
     # produced by python >=3.5 on execnet (pytest-xdist)