[issue2521] ABC caches should use weak refs

[Benjamin Peterson]

Benjamin Peterson <benjamin at python.org> added the comment:

Some comments:
- The test should be in test_abc.py and should probably not use a collections class.
- Use test_support.gc_collect().
- What's the __len__() stuff for?

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keywords:  -26backport

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<http://bugs.python.org/issue2521>
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