This requires some changes to the way we handle errors, specifically
for signals in the monkey-patched test_selectors.py and
test_socket.py.

It's not absolutely clear that this is the right thing to do in all
cases.

Fix #1044 by always closing opened sockets before raising

Account for some more warnings that Python 3.6 can generate: ImportWarning: can't resolve package from __spec__ or __package__, falling back on __name__ and __path__. See https://travis-ci.org/gevent/gevent/jobs/310040846

Also enable ResoureWarnings by default in the test suite and fix a
bunch that showed up.

Make universal newlines with FileObjectPosix return str on Python 2 i…

Upgrade c-ares to 1.13.0. Fixes #990

Enable warnings when running our tests. Rename (the undocumented) threadpool.ThreadResult.async to async_watcher because it's a keyword in 3.7, as exposed by warnings.