[3.13] gh-101100: Make __subclasses__ doctest stable (GH-124577) by miss-islington · Pull Request #124579 · python/cpython

Using a standard library class makes this test difficult to maintain
as other tests and other parts of the stdlib may create subclasses,
which may still be alive when this test runs depending on GC timing.
(cherry picked from commit 08a467b)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>