[issue17643] Expose weakref callback for introspection purposes.

[Richard Oudkerk]

Richard Oudkerk added the comment:

> I could mark the new test as cpython only, or add a gc.collect (but I'm 
> not sure if the latter is enough, either).

test.support.gc_collect() should work on non-refcounted implementations.

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nosy: +sbt

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<http://bugs.python.org/issue17643>
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