Current results:

| Benchmark             | gevent-libev-cext | gevent-libev-cffi             | gevent-libuv-cffi               |
|-----------------------|-------------------|-------------------------------|---------------------------------|
| gevent spawn          | 12.5 us           | 13.7 us: 1.10x slower (+10%)  | 14.2 us: 1.14x slower (+14%)    |
| gevent sleep          | 2.09 us           | 3.16 us: 1.51x slower (+51%)  | 74.4 us: 35.64x slower (+3464%) |
| geventpool sleep      | 3.46 us           | 6.20 us: 1.79x slower (+79%)  | 151 us: 43.58x slower (+4258%)  |
| geventraw spawn       | 5.34 us           | 6.54 us: 1.22x slower (+22%)  | 6.38 us: 1.19x slower (+19%)    |
| geventraw sleep       | 949 ns            | 1.66 us: 1.75x slower (+75%)  | 43.9 us: 46.25x slower (+4525%) |
| geventpool join       | 1.57 us           | 3.43 us: 2.18x slower (+118%) | 88.2 us: 56.07x slower (+5507%) |
| gevent spawn kwarg    | 14.2 us           | 12.4 us: 1.15x faster (-13%)  | 14.8 us: 1.04x slower (+4%)     |
| geventraw spawn kwarg | 7.68 us           | 8.38 us: 1.09x slower (+9%)   | 8.47 us: 1.10x slower (+10%)    |
| none spawn kwarg      | 771 ns            | 738 ns: 1.04x faster (-4%)    | 734 ns: 1.05x faster (-5%)      |

Not significant (3): geventpool spawn; none spawn; geventpool spawn kwarg

Refs #1493

Still needs better docs.

Improve safety for libuv async and idle watchers.

Fixes #1489.

gevent.pywsgi: Support keep-alive in HTTP/1.0

With tests. Supercedes #1331.

Remove redundant checks for constant values.

Make Semaphores fair.

Preserve order of wakeups. Take steps to avoid going back to the O(n^2) old behaviour, though, much like Greenlet does.

This means we auto-unlink links as we call them. Most users won't notice as that's a very low-level method, but baseserver.py was relying on the same rawlink being called every time a semaphore became ready. That was *a lot*, so this eliminates some calls.

coveralls 1.9 is incompatible with coverage >= 5.0a5

Fix #1464 and fix #1465 by making LockType sleep on failure to non-blocking acquire the lock.

Using sleep doesn't force a trip around the event loop until the switch interval elapses.

More simplification and unification of the file objects

Keeping control this way simplifies and unifies the implementations.

- Fix a concurrency bug in FileObjectPosix that we were hiding with buffering and a RuntimeError.
- Add all the error checking the stdlib does, with a few exceptions that make sense.

See #1441.

And add tests.

Fixes #1441.

Update Python versions, including 3.8 final on Appveyor

Fix pylint 2.4 new warnings.

Attempting to match Python 3.8 fixes in subprocess.py, especially on Windows.

Attempting to fix psutil-related regression in PyPy 7.2

corrected a typo in comments

* Fix tests with TLS1.3

There were hidden assumptions about the order in which greenlets would run.

TLS1.3 changed the handshake in a way that broke those assumptions.

* Attempting to fix the appveyor backdoor test timeouts.

Suspect a buffering issue. Trying to fix by disabling Nagle's algorithm.

Also a cleaner separation of concerns in the backdoor server.

* Brute-force debugging.

* Tweak the backlog and simultaneous accept settings, and drop the extra event listeners we don't need.

* max_accept=1 and dropping the events fixed test__backdoor on windows.

probably it was just the events. test that.

* Brute force debugging for the SSL issues.

* Some of the windows ssl issues may be due to fileno reuse and hence incorrectly multiplexed watchers?

* Even more care with socket lifetimes.

On Python 3.7, use the newer wrapping form that the stdlib uses to create the _sslobj.

* More debugging.

* The preferred method of unwrapping differs on 3.6 and 3.7+

* Checking ordering.

* Partial success, hopefully.

* Lots of progress. Try again.

* More Windows work.

* D'oh. Test cases from 3.7.4 handle some of these issues more gracefully so we will just use them.

* Tweaks.

* Having a hard time getting 3.7 updated on Travis. Try to force the issue.

* Yet another weird error from appveyor.

Add support for Python 3.8b4