Add pytest.ini from responder

This seems to be produce a more informative output, though I'm not
sure how to filter the annoying deprecation warnings pytest throws
about things that are usually done by modules I'm using, not by me.

From: https://github.com/taoufik07/responder/blob/master/pytest.ini

pytest.ini

@@ -0,0 +1,4 @@
+[pytest]
+addopts= -rsxX -s -v --strict
+filterwarnings =
+    error::UserWarning