Create fully pinned requirements file for bug-fix branches #23548
Description
I spent some time yesterday and today trying to get tests to run on old versions of Matplotlib which....was harder than I wished it would be. We may want to have one very pinned requirements file (possible managed by one of the auto-bump services 😞 or tools) that we rename to fully-pinned-v3.6.x.txt on branching. We can then use that (exclusively) for tests on the backport branch which should hopefully reduce the amount of dep-only backports we have to do (and provide a useful reference for anyone who wants to get old versions testing again).
Originally posted by @tacaswell in #22574 (comment)
melissawmHi all - we do not assign issues, so if the issue is open and does not already have an open PR associated with it, it is generally available for contribution. However, please try to coordinate if others have expressed interest in working on the issue recently, so you are not duplicating effort. If you have questions about the best approach for a solution, feel free to post an outline here so we can help.