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

This lets us use the natural location for .coveragerc

By not actually recording anything for the tests. Presumably because they're executed as scripts, not as gevent.tests.whatever.