More tweaks for test__fileobject.py on 3.7

Deterministically close GreenFileDescriptorIO backing FileObjectPosix if the (libuv) event loop watchers produce an error.

Especially on Python 3.7, the C-level garbage collection could kick in and close the underlying fileno at an inopportune time, killing an unrelated file, even though there were no Python-level references alive (apparently).

Cast OSError raised by os.lseek into IOError

This cleans up the skips needed.

Don't introduce a reference cycle with the traceback.

In the test, don't use sys.stdout as a non-seekable stream; putting it into non-blocking mode messes up the console. Instead create a pipe for this purpose.

Simplify test cases by splitting FOPosix and FOThread.

Make ThreadPool.join respect the loop.min_sleep_time value.

Setting the size was already doing so. By not using the hardcoded value, this prevents a UserWarning from the libuv backend.

Also rename the attribute to  and implement it for all loops.
Add this to the ILoop interface, and test that all loop implementations correctly implement it with zope.interface.verify.verifyObject.

Fixes #1321.

Python 2: Correct errors from async closed sockets.

We saw a Windows crash (https://ci.appveyor.com/project/denik/gevent/builds/20295148/job/ldltku6gletkdfgl):

| C:\Python37-x64\python.exe -u -mgevent.tests.test__os
461  .
462  ----------------------------------------------------------------------
463  Ran 1 test in 1.656s
464
465  OK
466  Windows fatal exception: access violation
467
468  Thread 0x000003dc (most recent call first):
469
470  Current thread 0x000005c8 (most recent call first):
471    File "C:\Python37-x64\lib\site-packages\gevent\threadpool.py", line 261 in _worker
472
473  Thread 0x00000af8 (most recent call first):
474    File "C:\Python37-x64\lib\site-packages\gevent\_threading.py", line 84 in wait
475    File "C:\Python37-x64\lib\site-packages\gevent\_threading.py", line 166 in get
476    File "C:\Python37-x64\lib\site-packages\gevent\threadpool.py", line 270 in _worker
477
478  Thread 0x00000424 (most recent call first):
479    File "C:\Python37-x64\lib\site-packages\gevent\_threading.py", line 84 in wait
480    File "C:\Python37-x64\lib\site-packages\gevent\_threading.py", line 166 in get
481    File "C:\Python37-x64\lib\site-packages\gevent\threadpool.py", line 270 in _worker
482
483  Thread 0x000003dc (most recent call first):
484    File "C:\Python37-x64\lib\weakref.py", line 356 in remove
485
486! C:\Python37-x64\python.exe -u -mgevent.tests.test__os [code 3221225477] [took 1.7s]

I confirmed that the one test we run there doesn't create an
IdentRegistry. Instead, the WeakKeyDictionary that's being created
comes from either zope.interface (tracking dependents) or
concurrent.futures (tracking spawned threads).

Previously we would create a AttributeError, instead of the preferred socket.error(EBADF).

Python 3 already did the right thing.

Discovered because of bad test coverage in test__hub.py that wasn't doing what we thought.

Incremental work on increasing test coverage.

Appveyor updates: Test PyPy6

Fixes #1314

Refactoring to run tests from an install

Also some testrunner improvements.

Also some debugging for #1296.

Set the non-standard wsgi.input_terminated flag. Fixes #1308.

Make gevent.socket.getaddrinfo return enums on Python 3

Fixes #1310

Refactoring greentest

Fixes #1167 and fixes #1168