Set EV_USE_REALTIME, EV_USE_MONOTONIC and EV_USE_FLOOR.

Fixes #1648

https://travis-ci.org/github/gevent/gevent/jobs/700121977

PyPy is failing with https://travis-ci.org/github/gevent/gevent/jobs/699973435

pip is now emitting a warning on appveyor, which appears to break powershell installing twine, and hence uploads.

Add libuv linked sources for Cygwin builds.  Fixes #1578

Relax the psutil version pin for PyPy.

5.7.0 works on Linux, but it still won't build on Windows.

Fixes #1643

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.