Do this by importing dns specially with sys.modules containing the
gevent modules. This is the technique eventlet uses (but we don't
leave the patched modules sitting around in sys.modules after we're
done; eventlet may have good reason to do that, but this is not a
public API here and is specialized just for dns).

Cleanups for timeout.py docs.

Deal gracefully with passing None watchers to hub.cancel_wait, something that may be more common now that we try to destroy watchers ASAP for GC reasons. Fixes #1089.

Add a DNSpython resolver

Bump Travis Python versions

[skip appveyor]

Drop the unused gevent._socket3._fileobject class. Fixes #1084.

* Massively simplify the internal _threading.py by removing everything we don't use.

Enable coverage testing for Python 3 and some for PyPy.

* PyPy can't handle coverage :(

* Account for a racy test and tweak the makefile to ignore errors uploading to coveralls. Python 3.7 was seen to generate that: https://travis-ci.org/gevent/gevent/jobs/334831358

* Manually exclude .so files. They caused coveralls to fail: https://travis-ci.org/gevent/gevent/jobs/334839765#L1755

* Enable coverage on pypy too.

* coverage pragmas

* Skip a test that fails under coverage sometimes

Allow all keyword arguments to Popen under Python 2.

And make restore_signals default to False on Python 2.

Fixes #1063.

Try to adhere to scheduling deadlines when running callbacks

[skip ci]

This should result in the loop getting serviced more often when there
are many callbacks to run. This is hooked up to `getswitchinterval` to
allow control.

We check the interval every 50 callbacks. That's hardcoded, which I
don't like, but the 1000 callback number was hardcoded before, so it's
not really much of a change. A future extension would be to calculate
the check number dynamically based on dividing the switch interval.

I don't have a specific test case for this change because I'm a little
fried, but all the other tests pass. I did manually verify that the
example in #1071 matches expectations---that could probably be turned
into a test case with some work.

Fixes #1072
Fixes #1071
Fixes #1068

Make callbacks.c stop relying on implementation details of Cython and Python. Instead use documented ways to interact.

Revert the hack of copying the ares.h header distributed with c-ares to cares.h. Our ares.pyx's channel class doesn't need to be public.

Cleanup a mistake in test_httpservers.py, and make a change to the example portforwarder to better handle race conditions.