The new Pickler reducer_override mechanism introduced in `Python3.8` generates a reference cycle: for optimization purposes, a the pickler.reducer_override bound method is referenced into the reducer_override attribute of the Pickler's struct. Thus, until as a gc.collect call is performed, both the Pickler and all the elements it pickled (as they are part of its memo), wont be collected.

We should break this cycle a the end of the dump() method.

See reproducer below:
```
import threading
import weakref
import pickle
import io


class MyClass:
    pass


my_object = MyClass()
collect = threading.Event()
_ = weakref.ref(my_object, lambda obj: collect.set())  # noqa


class MyPickler(pickle.Pickler):
    def reducer_override(self, obj):
        return NotImplemented


my_pickler = MyPickler(io.BytesIO())
my_pickler.dump(my_object)
del my_object
del my_pickler

# import gc
# gc.collect()

for i in range(5):
    collected = collect.wait(timeout=0.1)
    if collected:
        print('my_object was successfully collected')
        break
```

New changeset 0f2f35e15f9fbee44ce042b724348419d8136bc5 by Pierre Glaser in branch 'master':
bpo-39492: Fix a reference cycle between reducer_override and a Pickler instance (GH-18266)
https://github.com/python/cpython/commit/0f2f35e15f9fbee44ce042b724348419d8136bc5

New changeset 17236873392533ce0c5d7bbf52cbd61bca171c59 by Antoine Pitrou in branch '3.8':
[3.8] bpo-39492: Fix a reference cycle between reducer_override and a Pickler instance (GH-18266) (#18316)
https://github.com/python/cpython/commit/17236873392533ce0c5d7bbf52cbd61bca171c59

Thank you for the PR!