minor doc fix

Support and require greenlet 0.4.17

The behaviour for ContextVar[cls] changed in some minor patch release of 3.8 and 3.7, and we have too old versions on appveyor.

Fixes #1656

Also disable 32-bit manylinux builds because of https://github.com/cython/cython/issues/3840

Handle SSLEOFError during sock.unwrap() from versions of Python with https://bugs.python.org/issue31122

The unpinned 3.9-dev build caught this.

Fixes #1674

Fix issue 1659

Fixes #1659 and fixes #1661

Memory usage of SSLContext goes down from 416B to 128B after patching.

Fixes #1654

Correct a typo in testing/__init__.py

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