Update pytest warningsfilter.

Author: domdfcoding

tox.ini

@@ -382,6 +382,7 @@ filterwarnings =
     always:The hookimpl pytest_configure uses old-style configuration options:pytest.PytestDeprecationWarning
     always:mypy_extensions.TypedDict is deprecated, and will be removed in a future version. Use typing.TypedDict or typing_extensions.TypedDict instead.:DeprecationWarning
     always:The `docutils.parsers.rst.directive.html` module will be removed in Docutils 2.0. Since Docutils 0.18, the "Meta" node is defined in `docutils.parsers.rst.directives.misc`:DeprecationWarning
+    always:The frontend.OptionParser class will be replaced by a subclass of argparse.ArgumentParser in Docutils 0.21 or later.:DeprecationWarning
 markers = sphinx