Always check for __dictrefoffset__ and __weakoffset__ in dataclasses with slots

[sobolevn]

Bug report

See #118099 (comment) and #118033

We now correctly check for __dictoffset__ and __weakoffset__ on C types without __slots__ defined:

cpython/Lib/dataclasses.py

Lines 1201 to 1212 in c68acb1

	match cls.__dict__.get('__slots__'):
	# `__dictoffset__` and `__weakrefoffset__` can tell us whether
	# the base type has dict/weakref slots, in a way that works correctly
	# for both Python classes and C extension types. Extension types
	# don't use `__slots__` for slot creation
	case None:
	slots = []
	if getattr(cls, '__weakrefoffset__', -1) != 0:
	slots.append('__weakref__')
	if getattr(cls, '__dictrefoffset__', -1) != 0:
	slots.append('__dict__')
	yield from slots

But, not in this case:

cpython/Lib/dataclasses.py

Lines 1213 to 1218 in c68acb1

	case str(slot):
	yield slot
	# Slots may be any iterable, but we cannot handle an iterator
	# because it will already be (partially) consumed.
	case iterable if not hasattr(iterable, '__next__'):
	yield from iterable

So, in theory there might be C types with __slots__ and __dictoffset__ and __weakoffset__.

I will investigate!