The previous implementation had multiple bugs:

• self intersection would result in nil set (set intersection_update, difference_update, symmetric_difference_update should check self is equal to given arg #3881, &= and -= operator between Set types do not work correctly. #3992 )
• Intersecting {1, 2, 3} in {1} and {2} would result in {1} instead of {}.

The new implementation uses the approach chosen by cpython, i.e., create a normal intersection and then replace self with the result.

Allows turning on one test in test_weakset. The equivalent test in test_set still fails due to some of the other set operations also being buggy on self application.