Use-after-free in `jsonyx.dump` mapping iteration via re-entrant key encoder

[nineteendo]

See python/cpython#142831

import jsonyx as json

entries = [("boom", object())]

class BadDict(dict):
    def items(self):
        return entries

def hook(obj):
    if isinstance(obj, str):
        entries.clear()

    return obj

json.dump(BadDict(), hook=hook)