import selectors inside Gunicorn

[benoitc]

For now selectors are only used inside the threaded worker and only work on Python 3. But with #792 and #874 we will need such feature available on python 2.7x as well. So I propose to import the selectors module in the Gunicorn codebase to avoid rewriting another but similar code.

Thoughts?

[tilgovi]

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