[issue21998] asyncio: support fork

[Yury Selivanov]

Yury Selivanov added the comment:

> A compromise for the short term would be to detect fork in debug mode
and raise an exception, and explain how to fix such bug. What do you
think?

I'd prefer to fix it properly in 3.7.

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<http://bugs.python.org/issue21998>
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