Suggest to use `frozendict` instead of `MappingProxyType`

[sobolevn]

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 that frozendict is preferable than MappingProxyType #152637

[rhettinger]

The doc style recommends a neutral stance. In general, the docs should be expressing preferences unless there are known significant harms. Which is better depends on the use case. The MappingProxyType is better for proxying existing dictionaries. Also, our own implementation of classes uses mappingproxy and a frozendict would not be a workable substitute.

Note these two do different things. One is a concrete container type that actually holds data that cannot be changed. The other is a wrapper around a mapping that renders the mapping immutable to the viewer of the proxy without affecting the underlying container.

It is a false statement to say the one is always better than the other (eventhough that may be the personal view and preference of the person who implemented frozendict).