gh-154594: Fix copy.deepcopy() re-copying objects whose __deepcopy__ returns None

[sreehariannam]

Fixes gh-154594.

copy.deepcopy() uses memo.get(d, None) to check the memo dict, treating None both as the "not cached" sentinel and as a value a __deepcopy__ can legitimately return. When an object's __deepcopy__ returns None, the memo hit is indistinguishable from a miss, so the object gets deep-copied again on every subsequent reference instead of once:

import copy

call_count = 0

class C:
    def __deepcopy__(self, memo):
        global call_count
        call_count += 1
        memo[id(self)] = None
        return None

obj = C()
copy.deepcopy([obj, obj, obj])
print(call_count)  # 3, expected 1

This was introduced by gh-132657 / GH-138429, which replaced the previous _nil = [] sentinel with None specifically because None is an immortal singleton in CPython, avoiding refcount contention under free-threading that a plain sentinel object would reintroduce.

Rather than reintroducing a sentinel object (which would bring back that contention) or special-casing None, this switches the lookup to memo[d] guarded by try/except KeyError, matching the pattern already used a few lines down in _deepcopy_tuple and _deepcopy_frozendict. This distinguishes every possible cached value (including None) from a genuine cache miss without any shared sentinel object.

Added a regression test and a changelog entry.