Occasional TypeError in threading code

[mpenkov]

This happens occasionally during our CI builds:

test_accepts_boto3_bucket (smart_open.tests.test_s3.IterBucketTest) ... Exception in thread Thread-3:
Traceback (most recent call last):
  File "/opt/python/2.7.14/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/opt/python/2.7.14/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/opt/python/2.7.14/lib/python2.7/multiprocessing/pool.py", line 392, in _handle_results
    task = get()
TypeError: __init__() takes exactly 3 arguments (2 given)