Make selectors2 truly optional on Py2 with dnspython

Fixes #1641

Fix docstrings for the derived classes needed to get atomic semaphore behaviour in pure-python mode.

Fixes #1639

This should save a minute or so.

Run on Ubuntu Bionic

On Ubuntu Bionic in CI, we see the system resolver return fewer results than our other resolvers, so just require a matching subset.

Also drop libev's verification level. Static asserts that crash the process ARE NOT HELPFUL.

Fixes for Fedora Rawhide and SSL

And tweak the way we find open connections on linux; psutil considers some things closed even when the fd is still valid.

Fixes #1637

On travis, it's complaining that: WARNING: The directory '/cache/pip' or its parent directory is not owned or is not writable by the current user. The cache has been disabled. Check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.

Fixes #1627

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.