Python 3.15b1 ABI support

[ngoldbaum]

towards fixing #6012.

Adds support for the PEP 820 PySlot API, updates the PEP 793 implementation, and adds the critical section API to the limited API. Also fixes a small change to PyTypeObject that crept in since alpha8.

Claude helped a bit for code review and to figure out the PySlot_FUNC and the __pyo3_pymodexport macros.

Ping @bschoenmaeckers because you asked in the issue what this would look like.

[codspeed-hq]

Merging this PR will not alter performance

✅ 125 untouched benchmarks

---

_{Comparing ngoldbaum:pep-820 (26809b9) with main (58d5a1e)}

[ngoldbaum]

I added 3.15 and 3.15t CI to the PR CI builds and the new CI is passing. I just rebased and squashed to get a better coverage report. (EDIT: I guess it makes sense the coverage CI is failing, given the new untested FFI definitions)

That said, this is definitely ready for review. Please let me know if you'd prefer I do this in smaller chunks.

[davidhewitt]

Thanks, overall looks good to me, a few small suggestions.

[davidhewitt]

Suggested change

	panic!("Slot ID out of range for u16!");
	panic!("Slot ID out of range for u16");

[clin1234]

Maybe it's just me, but perhaps we should maintain a PyO3-specific fork of bindgen to output better type names for anonymous unions and other kludges with for_all_fields...

[ngoldbaum]

OK, ready for another review pass.

[clin1234]

Suggested change

	pub const fn PySlot_STATIC_DATA(NAME: c_int, VALUE: *mut c_void) -> PySlot {
	PySlot {
	sl_id: safe_cast_c_int_to_u16(NAME),
	sl_flags: PySlot_STATIC,
	anon1: _anon_union_32b { sl_reserved: 0 },
	anon2: _anon_union_64b { sl_ptr: VALUE },
	}
	pub const fn PySlot_STATIC_DATA(NAME: c_int, VALUE: std::ptr::NonNull<*mut c_void>) -> PySlot {
	PySlot {
	sl_id: safe_cast_c_int_to_u16(NAME),
	sl_flags: PySlot_STATIC,
	anon1: _anon_union_32b { sl_reserved: 0 },
	anon2: _anon_union_64b { unsafe {
	sl_ptr: (*VALUE).as_ptr()
	} },
	}

[clin1234]

Suggested change

	pub const fn PySlot_PTR(NAME: c_int, VALUE: *mut c_void) -> PySlot {
	PySlot {
	sl_id: safe_cast_c_int_to_u16(NAME),
	sl_flags: PySlot_INTPTR,
	anon1: _anon_union_32b { sl_reserved: 0 },
	anon2: _anon_union_64b { sl_ptr: VALUE },
	}
	pub const fn PySlot_PTR(NAME: c_int, VALUE: std::ptr::NonNull<*mut c_void>) -> PySlot {
	PySlot {
	sl_id: safe_cast_c_int_to_u16(NAME),
	sl_flags: PySlot_INTPTR,
	anon1: _anon_union_32b { sl_reserved: 0 },
	anon2: _anon_union_64b { unsafe {
	sl_ptr: (*VALUE).as_ptr()
	} },
	}

[clin1234]

Suggested change

	pub const fn PySlot_PTR_STATIC(NAME: c_int, VALUE: *mut c_void) -> PySlot {
	PySlot {
	sl_id: safe_cast_c_int_to_u16(NAME),
	sl_flags: PySlot_INTPTR | PySlot_STATIC,
	anon1: _anon_union_32b { sl_reserved: 0 },
	anon2: _anon_union_64b { sl_ptr: VALUE },
	}
	}
	pub const fn PySlot_PTR_STATIC(NAME: c_int, VALUE: std::ptr::NonNull<*mut c_void>) -> PySlot {
	PySlot {
	sl_id: safe_cast_c_int_to_u16(NAME),
	sl_flags: PySlot_INTPTR | PySlot_STATIC,
	anon1: _anon_union_32b { sl_reserved: 0 },
	anon2: _anon_union_64b { unsafe {
	sl_ptr: (*VALUE).as_ptr()
	} },
	}
	}

[davidhewitt]

Just two minor things

[ngoldbaum]

I'm not sure whether it's correct to work around clippy warning that PySlot_FUNC should be marked unsafe because the NULL check happens conditionally. I just went ahead and marked PySlot_FUNC as unsafe for now. We can always relax that later.