Suggest to use frozendict instead of MappingProxyType #152636

As suggested by @vstinner in #152483 (review) we should add a note in the MappingProxyType's docs that there are different potential problems with this type on older versions of Python: #152405

And that we recommend using frozendict on 3.15+

Linked PRs

• gh-152636: Add a note about frozendict to the types.MappingProxyType #152637