[WIP] bpo-36560: _abc now uses weakref.WeakSet type

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The C accelerator _abc now uses the weakref.WeakSet type rather than
the set type holding weak references.

Changes:

• _abc._get_dump() now returns lists rather than sets.
• subclasscheck_check_registry() now longer holds a second strong
  reference to 'rkey' to call PyObject_IsSubclass(): the 'registry'
  list already holds a strong reference.

https://bugs.python.org/issue36560

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@ilevkivskyi: I wrote this PR to try to debug https://bugs.python.org/issue36560

That's also why I had to fix regrtest when _abc is not built: https://bugs.python.org/issue36565

Honestly, I'm not sure that using weakref.WeakSet type provides any benefit compared to the current implementation, but I wanted to show you the code just in case :-)

I'm also using this PR to debug https://bugs.python.org/issue36560 even if modifying Modules/Setup to disable the compilation of the _abc module should have the same effect: Lib/_py_abc.py already uses weakref.WeakSet, so it should provide the same behavior than this PR.

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Honestly, I'm not sure that using weakref.WeakSet type provides any benefit compared to the current implementation, but I wanted to show you the code just in case :-)

Moreover, I didn't measure the performance impact yet. Lib/_weakset.py is implemented fully in pure-Python: it might be slower than your efficient implementation written in C.

[ilevkivskyi]

Yes, WeakSet was considered initially when we worked on this. IIRC we decided to not do this because it was slow. Also TBH it doesn't add much in readability. Anyway, thanks for debugging the refleaks!

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Please keep this PR open until https://bugs.python.org/issue36560 is closed.

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This PR isn't really needed to debug https://bugs.python.org/issue36560 : modify Modules/Setup to no longer compile "_abc" does basically the same thing than this PR.