gh-105927: _abc and _thread use PyWeakref_GetRef() (#105961)

Hold a strong reference on the object, rather than using a borrowed reference:
replace PyWeakref_GET_OBJECT() with PyWeakref_GetRef() and
_PyWeakref_GET_REF().

Remove assert(PyWeakref_CheckRef(localweakref)) since it's already
tested by _PyWeakref_GET_REF().