Add gevent.util.assert_switches 

Based on refactoring existing code in _monitor.py

Flexibly allows checking for any switching, switches that take longer
than a set amount of time, or switches that exceed a time limit
between getting back to the hub.

Fixes #1182.

Update to libuv 1.20.1

Update libuv to 1.20

We're more competitive using a forked process:

.....................
gevent socketpair sendall greenlet: Mean +- std dev: 256 ms +- 6 ms
.....................
native socketpair sendall thread: Mean +- std dev: 44.4 ms +- 1.5 ms
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (7.31 ms) is 13% of the mean (57.9 ms)
* the maximum (89.0 ms) is 54% greater than the mean (57.9 ms)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

gevent socketpair sendall fork: Mean +- std dev: 57.9 ms +- 7.3 ms
.....................
native socketpair sendall fork: Mean +- std dev: 44.6 ms +- 2.0 ms
.....................
native udp sendto: Mean +- std dev: 30.6 ms +- 1.1 ms
.....................
gevent udp sendto: Mean +- std dev: 37.5 ms +- 2.7 ms

(python 3.7)

This addresses the first few points of #1060.

We now have a basic documentation for what a loop looks like, and we
have documented watcher.close()

Hub is better documented (only the parts that should be documented are
documented).

get/setswitchinterval is documnted.

Several xrefs are fixed up.

Sigh. Pip changed the semantics of -U in pip 10. go back to the old way so we're actually testing changing deps; if it breaks we need to know.

Use /dev/fd|/proc/self/fd to get open FDs to close in Popen

Should get them to complete coverage.

Also move our test dependencies into the standard 'test' extra and
install mock (and futures) only on Python 2.7.

If those aren't available, use the old brute-force approach. This is
closer to what CPython does in its C implementation, and is much
faster.

We don't have to worry about the async signal safe stuff the C code
does because, guess what, we're running Python code here already
anyway, so much of it could wind up doing something that's not
actually safe anyway. Oh well.

Since we depend on Python 3.4 and above now, we can rely on the
CLOEXEC flag being set by default and not have to manually check
everything.

This speeds up 2.7 (close_fds defaults to *false* there, so the
default case doesn't change):

| Benchmark                 | 27_bench_subprocess | 27_bench_subprocess_dirfd     |
+---------------------------+---------------------+-------------------------------+
| spawn native no close_fds | 1.81 ms             | 1.79 ms: 1.01x faster (-1%)   |
| spawn gevent no close_fds | 2.11 ms             | 2.20 ms: 1.04x slower (+4%)   |
| spawn native close_fds    | 31.0 ms             | 30.2 ms: 1.03x faster (-3%)   |
| spawn gevent close_fds    | 31.6 ms             | 2.56 ms: 12.31x faster (-92%) |

And it really speeds up 3.7 (close_fds defaults to *true* there, so
the default case is much faster, and the non-default case is even better):

| Benchmark                 | 37_bench_subprocess | 37_bench_subprocess_dirfd     |
+---------------------------+---------------------+-------------------------------+
| spawn native no close_fds | 1.34 ms             | 1.27 ms: 1.06x faster (-6%)   |
| spawn gevent no close_fds | 117 ms              | 3.05 ms: 38.27x faster (-97%) |
| spawn native close_fds    | 1.36 ms             | 1.30 ms: 1.04x faster (-4%)   |
| spawn gevent close_fds    | 32.5 ms             | 3.34 ms: 9.75x faster (-90%)  |

Fixes #1172

In response to #1172

The following numbers are for my machine on macOS 10.13.3 with MAXFD
of 50000.

Python 2.7:

.....................
spawn native no close_fds: Mean +- std dev: 1.81 ms +- 0.04 ms
.....................
spawn gevent no close_fds: Mean +- std dev: 2.11 ms +- 0.08 ms
.....................
spawn native close_fds: Mean +- std dev: 31.0 ms +- 0.7 ms
.....................
spawn gevent close_fds: Mean +- std dev: 31.6 ms +- 0.6 ms

Notice that the times when close_fd=True (not the default on 2.7) are
about the same. 2.7 uses the same Python loop we do to close all the
fds.

Now 3.7:

.....................
spawn native no close_fds: Mean +- std dev: 1.34 ms +- 0.04 ms
.....................
spawn gevent no close_fds: Mean +- std dev: 117 ms +- 2 ms
.....................
spawn native close_fds: Mean +- std dev: 1.36 ms +- 0.03 ms
.....................
spawn gevent close_fds: Mean +- std dev: 32.5 ms +- 0.4 ms

Notice that gevent is *much* slower when we *don't* close the fds.
This is because, starting in Python 3.4, close_fds defaults to true,
and when it's false we have to check os.get_inheritable() for each fd
before we close it. gevent performs the same as it did on Python 2.7
when closing fds, but the native implementation is much faster due to
the C optimizations outlined in #1172---it turns out they apply to BSD
and Apple platforms in addition to Linux, although they're not async
safe.

Now, the C code does the opposite for inheritable handles: it
explicitly calls make_inheritable() for the ones it wants to keep and
lets the OS close the others with CLOEXEC. We could probably do that
too; the slow down for this case counts as a regression, I think.

Move the API reference to its own (sectioned) page to clean up the main page. Break up the massive 'gevent' module page into more digestable parts and add more xrefs.

[skip ci]

Use environment markers to install CFFI on windows so the libuv backend can really be default

Also try working around all the failed builds that pip 10 created: https://ci.appveyor.com/project/denik/gevent/build/1.0.1667/job/9jarsdxb6y0x2smd

[skip ci]

[skip ci]

[skip ci]

Add extension points to gevent.monkey using events and setuptools entry points.

Fixes #1162. Refs #1158.

Make libuv the default backend on Windows. Fixes #1163.

[skip ci]