Re-implement multiprocessing for macOS

[jdesboeufs]

Since Python 3.8, batch indexing is broken on macOS.

multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
  File "/Users/jerome/.pyenv/versions/3.10.7/lib/python3.10/multiprocessing/pool.py", line 125, in worker
    result = (True, func(*args, **kwds))
  File "/Users/jerome/.pyenv/versions/3.10.7/lib/python3.10/site-packages/addok/autocomplete.py", line 112, in index_ngram_keys
    pipe = DB.pipeline(transaction=False)
  File "/Users/jerome/.pyenv/versions/3.10.7/lib/python3.10/site-packages/addok/db.py", line 18, in __getattr__
    return getattr(self.instance, name)
AttributeError: 'NoneType' object has no attribute 'pipeline'
"""

After some investigation, I learned multiprocessing had a new behavior:

Changed in version 3.8: On macOS, the spawn start method is now the default. The fork start method should be considered unsafe as it can lead to crashes of the subprocess. See bpo-33725.

(source: https://docs.python.org/3/library/multiprocessing.html)

I temporarily fixed the problem disabling multiprocessing on macOS (af47d75).
Not critical since macOS is nearly always a development environment.