Fixed multiprocessing issue on win64 with more than 60 cores

[inuik]

When running autoflake on win64 with more than 60 cores the following error is encountered

> autoflake -c -r .\src
Exception in thread Thread-1:
Traceback (most recent call last):
  File "c:\python\lib\threading.py", line 932, in _bootstrap_inner
    self.run()
  File "c:\python\lib\threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "c:\python\lib\multiprocessing\pool.py", line 519, in _handle_workers
    cls._wait_for_updates(current_sentinels, change_notifier)
  File "c:\python\lib\multiprocessing\pool.py", line 499, in _wait_for_updates
    wait(sentinels, timeout=timeout)
  File "c:\python\lib\multiprocessing\connection.py", line 879, in wait
    ready_handles = _exhaustive_wait(waithandle_to_obj.keys(), timeout)
  File "c:\python\lib\multiprocessing\connection.py", line 811, in _exhaustive_wait
    res = _winapi.WaitForMultipleObjects(L, False, timeout)
ValueError: need at most 63 handles, got a sequence of length 66

The issue seems to have already affected projects like Black, see https://github.com/psf/black/pull/1912/files, this pull request is simply porting the same fix.

[fsouza]

Thanks for fixing. I want 64 cores too x)

[inuik]

I wish I had it for myself too, but it's actually a cloud based runner.

Thanks for the quick merge & release!