There's a researcher, Jiang Yuancheng, who's doing a great work finding CPython crashes and memory leaks: https://github.com/python/cpython/issues?q=is%3Aissue%20author%3AYuanchengJiang
They've come up with a very clever idea for a new way of fuzzing, made a fine tool out of it, and are reaping great results.
Fuzzing can be a diminishing returns endeavor: you only have so many bugs to find. Their approach has shown itself to cover different areas and kinds of issues well, as shown by their track record.