Install openssl on travis for 3.7b1 as a fix for  https://github.com/travis-ci/travis-ci/issues/9069

[skip appveyor]

The build cache was the only thing keeping 3.7.0a4 around; it can no
longer be installed from pyenv. And I accidentaly deleted the master
build cache, so we have no choice.

It may be difficult to get 3.7b1 installed due to
https://github.com/travis-ci/travis-ci/issues/9069

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.