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The serve method of the DjangoStreamingServer class support
If-Last-Modified, and for this it requires a timestamp of the last
modified time of the private file. Python 3 datetimes have a
timestamp method, which is used to generate the required timestamp,
but this isn't available in Python 2. Serving files on S3 therefore
fails if Python 2 is used.

This commit adds an alternative means of generating the timestamp
if Python 2 is used.