RFC: treat PyTypeObject as an opaque struct for Limited API compliance

[neutrinoceros]

needed for #151
based off #153

This one doesn't compile yet, but the good news is that the errors are identical wether I force abi3 or not.

extension/wrapper.cpp:124:15: error: cannot initialize a variable of type 'allocfunc' (aka '_object *(*)(_typeobject *, long)') with an rvalue of type 'void *'
  124 |     allocfunc alloc_func = PyType_GetSlot(cls, Py_tp_alloc);
      |               ^            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
extension/wrapper.cpp:514:15: error: cannot initialize a variable of type 'allocfunc' (aka '_object *(*)(_typeobject *, long)') with an rvalue of type 'void *'
  514 |     allocfunc alloc_func = PyType_GetSlot(type, Py_tp_alloc);
      |               ^            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

[neutrinoceros]

The only difference I can see between this broken code and the known-working examples I have in astropy is that this module is C++ while the ones I can compile are C... if that's the meaningful difference I may be out of my depth, as I cannot think of a single reason why that would matter.

[neutrinoceros]

found inspiration from this stack overflow thread. It compiles okay if I use type casting. Maybe C compilers just do the exact same thing implicitly and I never knew it was needed ?

[brandon-rhodes]

I think I only spotted one naming tweak, then we can get this merged!