Closes #12860.

isset() was checking isinstance(x, set | frozenset), so dict.items() and dict.keys() — which both implement collections.abc.Set — never triggered the set comparison path. The result was a raw repr diff with no useful information:

E  AssertionError: assert dict_items([('a', 1), ('b', 2)]) >= dict_items([('a', 1), ('b', 2), ('c', 3)])
E   +  where dict_items(...) = {'a': 1, 'b': 2}.items()
E   +  and   dict_items(...) = {'a': 1, 'b': 2, 'c': 3}.items()

After this fix:

E  AssertionError: assert dict_items([('a', 1), ('b', 2)]) >= dict_items([('a', 1), ('b', 2), ('c', 3)])
E    Extra items in the right set:
E    ('c', 3)

The fix is to widen isset() to isinstance(x, collections.abc.Set). This covers set, frozenset, dict.items(), and dict.keys() — all of which support - and the comparison operators that the set assertion path relies on. The existing _compare_set.py functions already type their parameters as AbstractSet, so no changes there were needed.