Fix python implementation pickling

[apmorton]

This PR is split into two commits for easier review since git doesn't correctly show the rename operation on core.py 😞 .

The first moved the pure python implementation to _frozendictpy so that core.py can become a compatibility wrapper, but makes no other changes. This allows existing pickles that mention frozendict.core.frozendict to correctly depickle as the native implementation in the future on python 3.11.

The second implements the fix for #87.

[Marco-Sulla]

Excuse me, but why you have renamed also core.py? Fixing #87 was not sufficient?

PS: I added a build and test for the pure py implementation in the pipeline, so I'm going to remove the pure py tests from the C extension tests.

Wait for it.

[apmorton]

Excuse me, but why you have renamed also core.py?

I explained this in the PR description.

Existing files that have been pickled on python 3.11 mention frozendict.core.frozendict.
This means that if/when 3.11 gets a native extension these files will still depickle to the pure python implementation and you can end up with mixed native/python instances in a single process.

Moving the python implementation and setting up frozendict.core.frozendict to be an alias for frozendict.frozendict resolves this issue.

[Marco-Sulla]

This means that if/when 3.11 gets a native extension these files will still depickle to the pure python implementation and you can end up with mixed native/python instances in a single process.

Ok, but what if I pickle frozendict._frozendictpy.frozendict and then I switch to the C extension?

At this point, is it not much simpler to remove core.py?

[apmorton]

Ok, but what if I pickle frozendict._frozendictpy.frozendict and then I switch to the C extension?

If you pickle frozendict._frozendictpy.frozendict it will appear in the pickle file as frozendict.frozendict because it has __module__ set to frozendict and depickle correctly in the future using the C extension.