gh-101100: Make __subclasses__ doctest stable (#124577)

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.

Author: JelleZijlstra

Doc/reference/datamodel.rst

@@ -1109,8 +1109,10 @@ have the following two methods available:
 
    .. doctest::
 
-      >>> int.__subclasses__()
-      [<class 'bool'>, <enum 'IntEnum'>, <flag 'IntFlag'>, <class 're._constants._NamedIntConstant'>, <class 're._ZeroSentinel'>]
+      >>> class A: pass
+      >>> class B(A): pass
+      >>> A.__subclasses__()
+      [<class 'B'>]
 
 Class instances
 ---------------