gh-141510: Add can_modify_dict() in dictobject.c (#144955)

can_modify_dict() is stricter than ASSERT_DICT_LOCKED() for
frozendict. It uses PyUnstable_Object_IsUniquelyReferenced() which
matters for free-threaded builds.

Replace anydict_setitem_take2() with setitem_take2_lock_held(). It's
no longer useful to have two functions.