s3parq parallel processing issue when fetching non-partitioned files

[RyanAdalbert]

Describe the bug

When retrieving a file that was published using s3parq.publish and that did not utilize partitioning, s3parq.fetch becomes stuck in an infinite error loop UNLESS the parallel=False is provided in the s3parq.fetch call.

To Reproduce

Code:

s3parq.publish(bucket=ENV_BUCKET, key=key_filename, partitions=[], dataframe=df_revised_files, redshift_params=None)
s3parq.fetch(bucket=ENV_BUCKET, key=key_filename, filters=[])

Stacktrace:

    df_existing_files = s3parq.fetch(bucket=ENV_BUCKET, key=key_filename, filters=[])
  File "/usr/local/lib/python3.6/site-packages/s3parq/fetch_parq.py", line 176, in fetch
    parallel=parallel)
  File "/usr/local/lib/python3.6/site-packages/s3parq/fetch_parq.py", line 383, in _get_filtered_data
    with get_context("spawn").Pool() as pool:
  File "/usr/local/lib/python3.6/multiprocessing/context.py", line 119, in Pool
    context=self.get_context())
  File "/usr/local/lib/python3.6/multiprocessing/pool.py", line 174, in __init__
    self._repopulate_pool()
  File "/usr/local/lib/python3.6/multiprocessing/pool.py", line 239, in _repopulate_pool
    w.start()
  File "/usr/local/lib/python3.6/multiprocessing/process.py", line 105, in start
    self._popen = self._Popen(self)
  File "/usr/local/lib/python3.6/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/usr/local/lib/python3.6/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/usr/local/lib/python3.6/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/usr/local/lib/python3.6/multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/usr/local/lib/python3.6/multiprocessing/spawn.py", line 143, in get_preparation_data
    _check_not_importing_main()
  File "/usr/local/lib/python3.6/multiprocessing/spawn.py", line 136, in _check_not_importing_main
    is not going to be frozen to produce an executable.''')
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.