Modernize macosx backend a bit

[QuLogic]

PR summary

Modify PyTypeObject definitions to match the 3.9 C-embedding tutorial. Use PyDoc_STR for possible optimization purposes. Re-order fields in a bit clearer manner (object information, new+init, dealloc, repr, methods). And finally, use the new PyModule_AddType, which does what our prepare_and_add_type inline function does.

PR checklist

• [n/a] "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• [n/a] New Features and API Changes are noted with a directive and release note
• [n/a] Documentation complies with general and docstring guidelines

[QuLogic]

The use of .ob_base = PyVarObject_HEAD_INIT as a struct initializer only appeared in 3.12 docs. I had hoped it would still work with 3.9, but apparently not, so I'll revert that bit.

[QuLogic]

Also, forgot to mention that I think PyTypeObject.tp_name should have been the fully-qualified name, not just the extension+type, as that is what we do in the other extensions.

[greglucas]

Python docs say it should be the fully qualified name including the package so I agree with that: https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_name

This looks good to me and the reordering makes sense.

A few other minor nit cleanups if you want to add them:

• The mplutils.h include is no longer needed either I don't think.
• We don't have any s# variants, so I don't think we need the #define PY_SSIZE_T_CLEAN either.

[QuLogic]

• The mplutils.h include is no longer needed either I don't think.

I removed this one; it was no longer needed due to dropping _prepare_and_add_type.

• We don't have any s# variants, so I don't think we need the #define PY_SSIZE_T_CLEAN either.

Yes, it's not needed, but I'd rather keep that one in case things change in the future.