Return whether the pool became empty in join.

Useful with a timeout argument.

Fixes #503.

[skip ci]

It would become False anyway as soon as the method returned. We won't
schedule another one while it's running (True), so there's no race
condition.

Switch to using prospector for lint checks

Don't immediately run links added to an Event/AsyncResult while it is running callbacks. Fixes #771.

and document the settings.

Fixed typo error

Figured out the root cause of the u-v-a crash in https://github.com/PyCQA/pylint/issues/846: unsafe-load-any-extension causes it.

cleanup threadpool.
various small cleanups.
cleanup _fileobjectposix.

clean up resolver_ares
cleanup thread.
cleanup testrunner
cleanup pool.
clean up event

clean up socketcommon.
cleanup examples and servers.
cleanup server
cleanup util.
cleanup sslgte279
cleanup greentest.
cleanup corecffi.
cleanup fileobject.
cleanup select.py
cleanup socket2
fix naming.
cleanup socket3
cleanup hub.
cleanup threading.
cleanup lock
cleanup pywsgi.
cleanup _threading
cleanup patched_test_setup.

On travis, start a build with 2.7.8 so that _ssl2.py gets
tested (previously tested by Python 2.6). Do so with the test_ssl.py
from that version (because the changes are large), but keep using the
2.7.11 tests with just a few removals otherwise.

Drop support for python 2.6.