[issue19465] selectors: provide a helper to choose a selector using constraints

[Charles-François Natali]

Charles-François Natali added the comment:

Of course, when I have 300 connections to remote nodes, I use poll()
to multiplex between them.

But there are times when you can have a large number of threads
running concurrently, and if many of them call e.g.
subprocess.check_output() at the same time (which does call
subprocess.communicate() behind the scene, and thus calls
select/poll), then one extra FD per instance could be an issue.
For example, in http://bugs.python.org/issue18756, os.urandom() would
start failing when multiple threads called it at the same time.

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