fix(deps): require six>=1.4.0

[busunkim96]

fix(deps): add lower bound for six
test: use constraints file to test lower bounds

[busunkim96]

From experimentation this was the lowest version that resulted in passing unit tests.

1.3.0 results in the following error:

_________________________________________ ERROR collecting tests_async/unit/requests/test_upload.py __________________________________________
Traceback (most recent call last):
  File "/usr/local/google/home/busunkim/github/google-resumable-media-python/tests_async/unit/requests/test_upload.py", line 23, in <module>
    import google._async_resumable_media.requests.upload as upload_mod
  File "/usr/local/google/home/busunkim/github/google-resumable-media-python/google/_async_resumable_media/requests/__init__.py", line 661, in <module>
    from google._async_resumable_media.requests.download import ChunkedDownload
  File "/usr/local/google/home/busunkim/github/google-resumable-media-python/google/_async_resumable_media/requests/download.py", line 21, in <module>
    from google._async_resumable_media.requests import _request_helpers
  File "/usr/local/google/home/busunkim/github/google-resumable-media-python/google/_async_resumable_media/requests/_request_helpers.py", line 26, in <module>
    import google.auth.transport._aiohttp_requests as aiohttp_requests
  File "/usr/local/google/home/busunkim/github/google-resumable-media-python/.nox/unit-3-6/lib/python3.6/site-packages/google/auth/__init__.py", line 19, in <module>
    from google.auth._default import default, load_credentials_from_file
  File "/usr/local/google/home/busunkim/github/google-resumable-media-python/.nox/unit-3-6/lib/python3.6/site-packages/google/auth/_default.py", line 30, in <module>
    import google.auth.transport._http_client
  File "/usr/local/google/home/busunkim/github/google-resumable-media-python/.nox/unit-3-6/lib/python3.6/site-packages/google/auth/transport/__init__.py", line 41, in <module>
    @six.add_metaclass(abc.ABCMeta)
AttributeError: module 'six' has no attribute 'add_metaclass'

add_metaclass was added in 1.4.0 https://github.com/benjaminp/six/blob/42636b15dd1a5b85de56eac98e47954d4c776576/CHANGES#L248-L249

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[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

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[parthea]

@googlebot I consent.

[tseaver]

Of course, we are going to be ripping six out altogether soon (fingers crossed, #229).