Feature: switch from select.select to selectors

[penguinolog]

• Use DefaultSelector as automatically selected most efficient.
• Use context manager for not holding resources.
• InterruptedError is handled by selector (and since python 3.5 by select.select, see PEP 475)

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 7140441898

• 19 of 22 (86.36%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.08%) to 72.57%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/_raw_display_base.py	0	1	0.0%
urwid/event_loop/select_loop.py	15	17	88.24%

Files with Coverage Reduction	New Missed Lines	%
urwid/event_loop/select_loop.py	1	88.6%

Totals
Change from base Build 7140121673:	0.08%
Covered Lines:	7579
Relevant Lines:	10558

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💛 - Coveralls

[penguinolog]

use modern api and parse self._watch_files once in iteration

[penguinolog]

we put callback here

[wardi]

reduces line count and probably makes SelectEventLoop perform better on linux, so that's great. This doesn't help with pipes not being selectable on windows though, right? Do all apps that want to be cross-platform need to be written to use sockets?

[penguinolog]

reduces line count and probably makes SelectEventLoop perform better on linux, so that's great. This doesn't help with pipes not being selectable on windows though, right? Do all apps that want to be cross-platform need to be written to use sockets?

Unfortunately, yes - only sockets under windows or use some gevent-like approach.
Technically we can use primitives with exchange without select for some operations, it was proposed long time ago. It will be not global solution, but reduce pain