RFC: redefine PyTypeObjects as heap types for Limited API compliance

[neutrinoceros]

based off #153
needed for #151

At the time of opening, I'm getting a single compilation error:

clang++: warning: optimization flag '-ffloat-store' is not supported [-Wignored-optimization-argument]
extension/wrapper.cpp:582:10: error: use of undeclared identifier 'Py_tp_as_sequence'
  582 |         {Py_tp_as_sequence, &SatrecArray_as_sequence},

I don't have experience with "sub slots", which seem needed for defining the tp_as_sequence slot correctly. Hopefully I can crack it soon but in the mean time, any help would be appreciated.

[neutrinoceros]

maybe the rabbit hole goes a lot deeper. I think this is just the first symptom of a much broader issue: SatrecArrayType is the first type of variable lenght that I attempt to convert to a heap type. I'm almost sure it should be possible, but I may be slow to reach a conclusion/solution.

[neutrinoceros]

FWIW I doubt that this change can be handled simply without dropping Python 2.7 too, but I also don't have a simple way to check.

[neutrinoceros]

... which might not be a problem since, IIUC, the extension is only ever compiled for Python 3 anyway

[neutrinoceros]

I got it to compile locally. Hopefully it runs smoothly in CI too.