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

It organizes things by the spawning greenlet, if possible.

Example output::

```
<greenlet.greenlet object at 0x10753a9b0>
 :    Running:
 :      File "/tmp/t.py", line 38, in <module>
 :        print("\n".join(format_run_info()))
 :      File "//src/gevent/util.py", line 99, in format_run_info
 :        _format_greenlet_info(lines)
 :      File "//src/gevent/util.py", line 132, in _format_greenlet_info
 :        lines.extend(tree.format_lines(details=True))
 :      File "//src/gevent/util.py", line 265, in format_lines
 :        for l in self._render(tree)]
 :      File "/-main/src/gevent/util.py", line 310, in _render
 :        self.__render_tb(tree, 'Running:', self.greenlet.gr_frame)
 :      File "//src/gevent/util.py", line 281, in __render_tb
 :        tb = ''.join(traceback.format_stack(frame))
 :    Greenlet Locals:
 :      Local <type 'gevent._local.local'> at 0x10759cec0
 :        {'foo': 42}
 +--- <Greenlet "Greenlet-0" at 0x107405cb0: _run>; finished with value <Greenlet "Greenlet-4" at 0x10
 :          Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :          Spawned at:
 :            File "/tmp/t.py", line 1, in <module>
 :              from gevent.util import GreenletTree
 |    +--- <Greenlet "Greenlet-4" at 0x10780b260: _run>; finished
 :                Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :                Spawned at:
 :                  File "/tmp/t.py", line 1, in <module>
 :                    from gevent.util import GreenletTree
 :                  File "/tmp/t.py", line 12, in t2
 :                    def t2():
 +--- <Hub at 0x10753a550 select default pending=0 ref=0>
 :          Parent: <greenlet.greenlet object at 0x10753a9b0>
 :          Running:
 :            File "/Users/jmadden/Projects/GithubSources/gevent-main/src/gevent/hub.py", line 673, in run
 :              loop.run()
 +--- <Greenlet "Greenlet-1" at 0x10780b368: _run>; finished with value <Greenlet "Greenlet-5" at 0x10
 :          Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :          Spawned at:
 :            File "/tmp/t.py", line 1, in <module>
 :              from gevent.util import GreenletTree
 |    +--- <Greenlet "Greenlet-5" at 0x10780b578: _run>; finished
 :                Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :                Spawned at:
 :                  File "/tmp/t.py", line 1, in <module>
 :                    from gevent.util import GreenletTree
 :                  File "/tmp/t.py", line 12, in t2
 :                    def t2():
 +--- <Greenlet "Greenlet-2" at 0x10780b470: _run>; finished with value <Greenlet "Greenlet-6" at 0x10
 :          Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :          Spawned at:
 :            File "/tmp/t.py", line 1, in <module>
 :              from gevent.util import GreenletTree
 :          Spawn Tree Locals
 :          {'stl': 'STL'}
 |    +--- <Greenlet "Greenlet-6" at 0x10780b680: _run>; finished with value <Greenlet "Greenlet-7" at 0x10
 :                Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :                Spawned at:
 :                  File "/tmp/t.py", line 1, in <module>
 :                    from gevent.util import GreenletTree
 :                  File "/tmp/t.py", line 21, in t3
 :                    def t3():
 |         +--- <Greenlet "Greenlet-7" at 0x10780b788: _run>; finished
 :                      Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :                      Spawned at:
 :                        File "/tmp/t.py", line 1, in <module>
 :                          from gevent.util import GreenletTree
 :                        File "/tmp/t.py", line 21, in t3
 :                          def t3():
 :                        File "/tmp/t.py", line 12, in t2
 :                          def t2():
 +--- <Greenlet "Greenlet-3" at 0x10780b890: _run>; finished with value [<gevent.util.GreenletTree obj
            Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
            Spawned at:
              File "/tmp/t.py", line 1, in <module>
                from gevent.util import GreenletTree
```

Include locals in the debugging output.

Also let greenlets have a name.

Some tweaks to the local implementation to speed it up by 3-4%.

Speed up Greenlet creation on CPython

Two ways: store tuples instead of _frame objects and use direct access
to two of the attributes of the CPython frame objects.

Benchmarks:

+------------------------+-----------------+------------------------------+
| Benchmark              | spawn_27_master | spawn_27_tuple2              |
+========================+=================+==============================+
| eventlet sleep         | 9.12 us         | 8.77 us: 1.04x faster (-4%)  |
+------------------------+-----------------+------------------------------+
| gevent spawn           | 14.5 us         | 13.2 us: 1.10x faster (-9%)  |
+------------------------+-----------------+------------------------------+
| gevent sleep           | 1.63 us         | 1.86 us: 1.14x slower (+14%) |
+------------------------+-----------------+------------------------------+
| geventpool spawn       | 30.4 us         | 23.6 us: 1.29x faster (-22%) |
+------------------------+-----------------+------------------------------+
| geventpool sleep       | 4.30 us         | 4.55 us: 1.06x slower (+6%)  |
+------------------------+-----------------+------------------------------+
| geventpool join        | 1.70 us         | 1.83 us: 1.08x slower (+8%)  |
+------------------------+-----------------+------------------------------+
| gevent spawn kwarg     | 16.5 us         | 13.5 us: 1.22x faster (-18%) |
+------------------------+-----------------+------------------------------+
| geventpool spawn kwarg | 30.5 us         | 23.9 us: 1.27x faster (-22%) |
+------------------------+-----------------+------------------------------+

Not significant (7): eventlet spawn; geventraw spawn; geventraw sleep;
none spawn; eventlet spawn kwarg; geventraw spawn kwarg; none spawn
kwarg

+------------------------+-----------------+------------------------------+
| Benchmark              | spawn_36_master | spawn_36_tuple2              |
+========================+=================+==============================+
| gevent spawn           | 13.2 us         | 11.9 us: 1.12x faster (-10%) |
+------------------------+-----------------+------------------------------+
| gevent sleep           | 1.71 us         | 1.90 us: 1.11x slower (+11%) |
+------------------------+-----------------+------------------------------+
| geventpool spawn       | 19.9 us         | 15.9 us: 1.25x faster (-20%) |
+------------------------+-----------------+------------------------------+
| geventpool sleep       | 3.54 us         | 3.75 us: 1.06x slower (+6%)  |
+------------------------+-----------------+------------------------------+
| geventpool spawn kwarg | 20.3 us         | 15.9 us: 1.27x faster (-22%) |
+------------------------+-----------------+------------------------------+
| geventraw spawn kwarg  | 5.80 us         | 6.10 us: 1.05x slower (+5%)  |
+------------------------+-----------------+------------------------------+

Not significant (9): eventlet spawn; eventlet sleep; geventraw spawn;
geventraw sleep; none spawn; geventpool join; eventlet spawn kwarg;
gevent spawn kwarg; none spawn kwarg

+------------------+-------------------+------------------------------+
| Benchmark        | spawn_pypy_master | spawn_pypy_tuple2            |
+==================+===================+==============================+
| eventlet spawn   | 30.5 us           | 28.9 us: 1.05x faster (-5%)  |
+------------------+-------------------+------------------------------+
| eventlet sleep   | 3.39 us           | 3.19 us: 1.06x faster (-6%)  |
+------------------+-------------------+------------------------------+
| gevent spawn     | 9.89 us           | 17.2 us: 1.73x slower (+73%) |
+------------------+-------------------+------------------------------+
| gevent sleep     | 3.14 us           | 3.99 us: 1.27x slower (+27%) |
+------------------+-------------------+------------------------------+
| geventpool spawn | 12.3 us           | 20.1 us: 1.63x slower (+63%) |
+------------------+-------------------+------------------------------+

Not significant (1): geventpool sleep

+------------------------+---------------+-------------------------------+
| Benchmark              | spawn_36_13a1 | spawn_36_tuple2               |
+========================+===============+===============================+
| eventlet spawn         | 14.0 us       | 13.2 us: 1.06x faster (-6%)   |
+------------------------+---------------+-------------------------------+
| gevent spawn           | 4.25 us       | 11.9 us: 2.79x slower (+179%) |
+------------------------+---------------+-------------------------------+
| gevent sleep           | 2.78 us       | 1.90 us: 1.46x faster (-32%)  |
+------------------------+---------------+-------------------------------+
| geventpool spawn       | 10.4 us       | 15.9 us: 1.52x slower (+52%)  |
+------------------------+---------------+-------------------------------+
| geventpool sleep       | 5.52 us       | 3.75 us: 1.47x faster (-32%)  |
+------------------------+---------------+-------------------------------+
| geventraw spawn        | 2.56 us       | 5.09 us: 1.99x slower (+99%)  |
+------------------------+---------------+-------------------------------+
| geventraw sleep        | 738 ns        | 838 ns: 1.14x slower (+14%)   |
+------------------------+---------------+-------------------------------+
| geventpool join        | 3.94 us       | 1.75 us: 2.25x faster (-56%)  |
+------------------------+---------------+-------------------------------+
| gevent spawn kwarg     | 5.50 us       | 12.1 us: 2.19x slower (+119%) |
+------------------------+---------------+-------------------------------+
| geventpool spawn kwarg | 11.3 us       | 15.9 us: 1.41x slower (+41%)  |
+------------------------+---------------+-------------------------------+
| geventraw spawn kwarg  | 3.90 us       | 6.10 us: 1.56x slower (+56%)  |
+------------------------+---------------+-------------------------------+

Not significant (4): eventlet sleep; none spawn; eventlet spawn kwarg; none spawn kwarg

The eventlet, sleep, join and raw tests serve as controls, so we can see
that there's up to ~10% variance between most runs anyway.

CPython 3.6 shows the least variance so those 10-20% improvement
numbers are probably fairly close.

PyPy sadly gets *slower* with this change for reasons that are utterly
unclear.

Compared to 1.3a1 (last benchmark) we're still up to 2-3x slower.

Creation of a raw greenlet shows 2.66us on CPython 3.6.4 vs the 3.65us
I reported in #755.