Filter out warnings caused by a bug in asyncio.

Those warnings make tests fail under PyPyy 3.9.

Refs #1483.

Author: aaugustin

tox.ini

@@ -12,7 +12,9 @@ env_list =
     mypy
 
 [testenv]
-commands = python -W error::DeprecationWarning -W error::PendingDeprecationWarning -m unittest {posargs}
+; Filter out ResourceWarnings to work around https://github.com/python/cpython/issues/90476.
+; This can be removed when dropping Python < 3.11.
+commands = python -W error::DeprecationWarning -W error::PendingDeprecationWarning -W ignore:"unclosed transport":ResourceWarning:asyncio.sslproto -m unittest {posargs}
 pass_env = WEBSOCKETS_*
 
 [testenv:coverage]