Before I realized that was the root cause, I refactored to avoid
starting the hub greenlet *at all*, and that also fixed the problem.

The problem was that some versions of CPython using the libev-cext
loop (and only that one) would crash garbage collecting a
greenlet (presumably the hub greenlet, which was attached to a
destroyed loop):

0   libsystem_kernel.dylib        	0x00007fff66455e3e __pthread_kill + 10
1   libsystem_pthread.dylib       	0x00007fff66594150 pthread_kill + 333
2   libsystem_c.dylib             	0x00007fff663648fe raise + 26
3   libsystem_platform.dylib      	0x00007fff66587f5a _sigtramp + 26
4   ???                           	000000000000000000 0 + 0
5   greenlet.cpython-35m-darwin.so	0x000000010d76b297 green_clear + 183
6   org.python.python             	0x000000010c707e0c collect + 1824
7   org.python.python             	0x000000010c7076e3 _PyGC_CollectNoFail + 43
8   org.python.python             	0x000000010c6e81d4 PyImport_Cleanup + 968
9   org.python.python             	0x000000010c6f1b49 Py_Finalize + 92
10  org.python.python             	0x000000010c6f20d5 Py_Exit + 13
11  org.python.python             	0x000000010c6f4631 handle_system_exit + 320
12  org.python.python             	0x000000010c6f42a1 PyErr_PrintEx + 42

We should take a look at that and see if we can do something more graceful.