[issue33623] Fix possible SIGSGV when asyncio.Future is created in __del__

[Yury Selivanov]

Yury Selivanov <yselivanov at gmail.com> added the comment:

There's nothing specific here to uvloop except the fact that it was compiled with Cython, so module finalization/GC pattern is slightly different to that of vanilla asyncio.

Serhiy, I'm merging this PR since I'd like to avoid having segfaults in 3.7.0.  Feel free to debug this further.

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