Add support for /etc/hosts to dnspython resolver.

Based on code from eventlet, but heavily refactored and modified:

- Split host parsing into its own class, re-used from the test case
- Hosts handle PTR lookups for ipv4
- Some bug fixes around what should be strings where.
- Allow strings for the rdtype paramater
- Watch last modified time instead of reloading the file at fixed
  intervals.

More of our DNS tests pass now (notable TestEtcHosts is now enabled
for this resolver), and we tend to generate more conformant results
than the ares resolver (by observation, not tested).

Let dnspython resolver be used without monkey-patching

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

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.