When bumping sphinx, remove FrameType workaround in docs/source/conf.py

[jakkdl]

github.com/sphinx-doc/sphinx/issues/11802 https://github.com/sphinx-doc/sphinx/pull/11861 being fixed should make https://github.com/python-trio/trio/blob/d1cc062bfe5e0c2fd7f3c702cc4390cbfdb904f0/docs/source/conf.py#L200-L203

redundant. Though I had some issues with getting TracebackType working with their workaround previously: https://github.com/python-trio/trio/blob/d1cc062bfe5e0c2fd7f3c702cc4390cbfdb904f0/src/trio/testing/_raises_group.py#L22-L24 that'll probably be the case with FrameType as well - and if so we should probably investigate and open an issue in sphinx about it.