Show warning about importing abcs from collections once

This is being raised by `unittest2.compatibility`:

DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

Author: nicoddemus

tox.ini

@@ -131,6 +131,7 @@ filterwarnings =
     ignore:yield tests are deprecated, and scheduled to be removed in pytest 4.0:pytest.RemovedInPytest4Warning
     ignore:Metafunc.addcall is deprecated and scheduled to be removed in pytest 4.0:pytest.RemovedInPytest4Warning
     ignore::pytest.RemovedInPytest4Warning
+    default:Using or importing the ABCs:DeprecationWarning:unittest2.*
     ignore:Module already imported so cannot be rewritten:pytest.PytestWarning
     # produced by path.local
     ignore:bad escape.*:DeprecationWarning:re