bpo-30570: fix an issubclass infinite recursion crash by gpshead · Pull Request #29017 · python/cpython

While this appears to work, I'd like to see the python benchmark suite run with it to make sure it doesn't have a notable performance impact. there are faster implementation alternatives. even just storing the visited pointers in a linear C array that we realloc when it needs to grow and doing a linear search in that or keeping it sorted for faster than O(n) lookup would be sufficient and waay less complex than using the PySet code.

#29048 is much simpler and nicer than this one. closing. :)