gh-141510: Fix copy.deepcopy() for recursive frozendict

[vstinner]

• Issue: PEP 814: Add built-in frozendict type #141510

[eendebakpt]

Good catch!

[serhiy-storchaka]

After adding the pickle support we may be able to remove the special deepcopy code.

[vstinner]

I added the tests requested by @serhiy-storchaka.

[vstinner]

After adding the pickle support we may be able to remove the special deepcopy code.

I propose merging #144967 PR first (once it will be approved), and then rework this PR. See also @serhiy-storchaka's comment on the pickle PR:

If things are pickleable, they should also be deepcopyable. It is worth to have explicit deepcopy tests, because they can preserve additional invariants. For example, it should be possible to deepcopy a frozendict containing lambdas or modules.

We can add more tests in this deepcopy PR.