fix: call 'PyObject_ClearWeakRefs' unconditionally

[tseaver]

Toward #323.

@stollero Could you test with this patch to see if it fixes #323?

[davisagli]

@tseaver Do we also need to call PyObject_ClearWeakRefs in the tp_dealloc functions for the other types that use BASETYPE_FLAGS?

[tseaver]

@davisagli

Do we also need to call PyObject_ClearWeakRefs in the tp_dealloc functions for the other types that use BASETYPE_FLAGS?

7ca6275 tries to comply across the board. PTAL.

[davisagli]

@tseaver The tests pass, but some errors are logged when I run tox -e py311:

Exception ignored in tp_clear of: <class 'dict'>
Traceback (most recent call last):
  File "/Users/davisagli/Plone/zope.interface/src/zope/interface/ro.py", line 289, in __init__
    def __init__(self, C, memo):

SystemError: Objects/weakrefobject.c:951: bad argument to internal function
Exception ignored in tp_clear of: <class 'dict'>
Traceback (most recent call last):
  File "/Users/davisagli/Plone/zope.interface/src/zope/interface/ro.py", line 289, in __init__
    def __init__(self, C, memo):

SystemError: Objects/weakrefobject.c:951: bad argument to internal function
...................................................Exception ignored in tp_clear of: <class 'dict'>
Traceback (most recent call last):
  File "/Users/davisagli/Plone/zope.interface/src/zope/interface/ro.py", line 247, in __init__
    def __init__(self, C, mro):

SystemError: Objects/weakrefobject.c:951: bad argument to internal function

That comes from here: https://github.com/python/cpython/blob/3.11/Objects/weakrefobject.c#L951

Apart from SB, the other types only have Py_TPFLAGS_MANAGED_WEAKREF (via BASETYPE_FLAGS) when USE_EXPLICIT_WEAKREFLIST is 0 (in Python 3.12+) so we should probably also use that condition for calling PyObject_ClearWeakRefs here.

Or else take Py_TPFLAGS_MANAGED_WEAKREF back out of BASETYPE_FLAGS and only add it for SpecificationBase