add support for abi3t

[Icxolu]

This adds abi3t support by

• switching to opaque structs
• exposing data access helpers from slots 369-378
• redirect all field accesses through the access helpers

Todo:

• changelog entry
• CI testing for 3.15/3.15t

See numpy/numpy#31091
Closes #555

[ngoldbaum]

Ping @kumaraditya303 - we were just talking about this today.

[neutrinoceros]

I confirmed that this resolved the runtime error I reported in #555
example logs (remaining failures are unrelated, they're just due to numpy not having cp315 nightlies for windows yet)

[kumaraditya303]

PyArray_DescrProto should not be exposed on abi3t as numpy does.

[kumaraditya303]

same for _PyArray_DescrNumPy2

[Icxolu]

I think __PyArray_DescrNumPy2 needs to be defined (with ob_base removed) as this is our (only) way to access the v2 specific fields.

[Icxolu]

For PyArray_DescrProto, what about slot 192 (PyArray_RegisterDataType) which takes it as an argument. Is it not available on abi3t and should be removed when building for it?

[kumaraditya303]

I think we should also add a check that when abi3t is used, the minimum numpy version should be 2.5.