Loky's _SafeQueue is not compatible with multiprocessing of Python 3.9 nightly.

[ogrisel]

As identified in #250.

Traceback (most recent call last):
  File "/home/vsts/work/1/s/loky/backend/popen_loky_posix.py", line 199, in <module>
    process_obj = pickle.load(from_parent)
  File "/home/vsts/work/1/s/loky/backend/queues.py", line 75, in __setstate__
    self._after_fork()
  File "/usr/lib/python3.9/multiprocessing/queues.py", line 69, in _after_fork
    self._reset(after_fork=True)
  File "/usr/lib/python3.9/multiprocessing/queues.py", line 73, in _reset
    self._notempty._at_fork_reinit()
AttributeError: '_SafeQueue' object has no attribute '_notempty'

[ogrisel]

A fix for this is going to be higher priority now that Python 3.9 beta1 is out.

[mgorny]

Ping. This is blocking us now.

[ogrisel]

Fixed by #250.