gh-141805: avoid increase the set->used twice when the __eq__ trigger another add

[kemingy]

gh-141805: goes directly to the found_unused_or_dummy when finding a dummy slot in set.add

This avoids triggering the __eq__ in the next iteration by the PyObject_RichCompareBool, which might introduce another set.add in the user code. Like:

tasks = set()

class Corrupt:
    def __eq__(self, value):
        tasks.add(self)
        return False

As described in #141805 (comment)