Add python-snappy to setup.py dependencies

[charlesccychen]

This change fixes an issue in postcommits caused by #5887, which depends on a newer version of python-snappy (which is a dependency of fastavro package), but did not update the setup.py file.

[charlesccychen]

R: @tvalentyn

[charlesccychen]

CC: @cclauss

[charlesccychen]

Run Python PostCommit

[charlesccychen]

Turns out there are issues with this change, e.g. below. Closing for now while investigating.

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ERROR: test_read_record_invalid_data_mask (apache_beam.io.tfrecordio_test.TestTFRecordUtil)
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Traceback (most recent call last):
  File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify_PR/src/sdks/python/apache_beam/io/tfrecordio_test.py", line 148, in test_read_record_invalid_data_mask
    self._test_error(record, 'Mismatch of data mask')
  File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify_PR/src/sdks/python/apache_beam/io/tfrecordio_test.py", line 102, in _test_error
    _TFRecordUtil.read_record(self._as_file_handle(record))
  File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify_PR/src/sdks/python/apache_beam/io/tfrecordio.py", line 125, in read_record
    length_mask_actual = cls._masked_crc32c(buf[:8])
  File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify_PR/src/sdks/python/apache_beam/io/tfrecordio.py", line 80, in _masked_crc32c
    crc = crc32c_fn(value)
  File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify_PR/src/sdks/python/apache_beam/io/tfrecordio.py", line 46, in _default_crc32c_fn
    _default_crc32c_fn.fn = snappy._crc32c  # pylint: disable=protected-access
AttributeError: 'module' object has no attribute '_crc32c'