Remove old warning filters (#398)

Author: jarrodmillman

setup.cfg

@@ -3,7 +3,3 @@ addopts =
     --showlocals --doctest-modules -ra --cov-report= --cov=numpydoc
     --junit-xml=junit-results.xml --ignore=doc/
 junit_family = xunit2
-filterwarnings =
-    ignore:'U' mode is deprecated:DeprecationWarning
-    ignore:.*sphinx\.util\.smartypants is deprecated.*:
-    ignore:Using or importing the ABCs.*:DeprecationWarning