Training not working on macOS due to multiprocessing queue missing implementation

[ggattoni]

Hi,
I'm struggling to run the training on macOS, while on windows everything works just fine.
The following error is raised when I run python -m alphazero.envs.tictactoe.train on a macOS terminal.

Traceback (most recent call last):
  File "/Users/User/anaconda3/envs/alphazero/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/Users/User/anaconda3/envs/alphazero/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/User/Projects/alphazero-general/alphazero/envs/tictactoe/train.py", line 37, in <module>
    c.learn()
  File "/Users/User/Projects/alphazero-general/alphazero/Coach.py", line 250, in learn
    self.saveIterationSamples(self.model_iter)
  File "/Users/User/Projects/alphazero-general/alphazero/Coach.py", line 146, in wrapper
    ret = func(self, *args, **kwargs)
  File "/Users/User/Projects/alphazero-general/alphazero/Coach.py", line 365, in saveIterationSamples
    num_samples = self.file_queue.qsize()
  File "/Users/User/anaconda3/envs/alphazero/lib/python3.9/multiprocessing/queues.py", line 126, in qsize
    return self._maxsize - self._sem._semlock._get_value()
NotImplementedError
/Users/User/anaconda3/envs/alphazero/lib/python3.9/multiprocessing/resource_tracker.py:216: UserWarning: resource_tracker: There appear to be 10 leaked semaphore objects to clean up at shutdown
  warnings.warn('resource_tracker: There appear to be %d '

I think that the issue is related to this issue.