Add gevent.selectors

Similar to ideas in https://github.com/gevent/gevent/pull/1523

We use it in our selector tests.

Fixes #1532, but there's potentially room for optimization. Committing the simple version for CI tests.

Also deprecate 'handle' as kwarg to WSGIServer. That's almost certainly not what anyone meant.

https://ci.appveyor.com/project/denik/gevent/builds/33296254/job/s630snlcgtqmq1fi

Pass addresses through inet_pton before getaddrinfo

The compiler for CPythno2 isn't there on the VS2019 image.

Especially on Windows, that could lead to FDs not being in the expected state. Account for this.

Mainly this is to force Appveyor to run.

It seems that  and  are now mutually exclusive.

The latter can lose information, like IPv6 scope IDs given in the string form.

Add support for Python 3.9

Begin trying to install on Travis.

That breaks several of the subprocess tests.

Catch greenlet.error when destroying a hub from another thread.

Also some test tweaks for OSX on Ci.

Fixes #1631

Fixes #1626

Update c-ares to 1.16.1