Use strongly typed watcher callback functions in libuv, preventing compiler warnings

Turns out we weren't passing the macros we thought we were at all. Whoops.

And also ensuring that in the Python code we know the type of pointer we're dealing with. This might catch some errors.

Fix documentation for Pool.full()

The original docs had the purpose of the boolean return value reversed. I fixed that distinction, and also tried to clarify how the method can be used.

Exclude 3.3, account for cython being a develop wheel right now.

[skip ci]

Run tests in 3.7 dev mode; cleanup resource warnings

Especially as reported on PyPy.

The testrunner will now show the output of a successful test if the
output contains ResourceWarning.

Fix a bunch of such warnings.

Import _testcapi in testrunner to try to avoid the concurrency issues on PyPy.

Update to 3.7.0b2

This matches what all versions of Python do and reverts
https://github.com/gevent/gevent/commit/9b1bccffc11455112076189f35023291cf97a2a2

The bug that commit references was marked as "won't fix". This is part
of an effort to adapt to changes in 3.7b2 that lead us to produce a
different errno than the system does for
`socket.create_connection(('localhost', ...))`

Interpret -1 to select.poll.poll as blocking, especially under libuv

Why are we bothering to run cares on PyPy? It has several known-bad tests, and it sometimes crashes the interpreter. We don't recommend or support it now that we have dnspython.

Fix #1127 by interpreting -1 as None under libuv for select.poll.poll; should probably start having Timeout do that itself.

Cherry pick 1ef16f38491000f4fdafdc01dc809d30fc785867

Add a GreenletTree for more organized, clearer output of greenlets