I think the problem is that the __del__ method fails on Windows, maybe because sys.stdout and sys.__stderr__ have been replaced by None.

Consider the following program:

  import os

  class C:
      def __del__(self, write=os.write):
          write(1, b"BEFORE\n")
          print("__del__ called")
          write(1, b"AFTER\n")

  l = [C()]
  l.append(l)

On Unix I get

  BEFORE
  __del__ called
  AFTER

but on Windows I only get

  BEFORE

I would suggest using os.write() instead of print() in the tests.