Its failing in a weird way that doesn't happen locally.

There is a remaining issue, which lies somewhere in greenlet, seen in
test__monkey_hub_in_thread and test__threading_monkey_in_thread:

  Fatal Python error: Python memory allocator called without holding the GIL

  Python runtime state: finalizing (tstate=0xd5f3e0)

  Thread 0x00007f511f66b700 (most recent call first):
  <no Python frame>

Obviously this mixes with threads. greenlet uses Py_AddPendingCall to
move some work to other threads, but apparently something is still
happening at the wrong time.