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