Updated patch according to the python-dev thread:

- processes renamed to workers
- `workers` defaults to 1
- When `workers` is equal to 0, then `os.cpu_count` will be used
- When `workers` > 1, multiple processes will be used
- When `workers` == 1, run normally (no multiple processes)
- Negative values raises a ValueError
- Will raise NotImplementedError if multiprocessing can't be used
(when `workers` equals to 0 or > 1)