This fixes the crash when attempting to create a new file in Python 3:

    $ gracedb Test gstlal HighMass coinc.xml
    Traceback (most recent call last):
      File "/Users/lpsinger/local/bin/gracedb", line 564, in <module>
        code = main()
      File "/Users/lpsinger/local/bin/gracedb", line 501, in main
        offline=options.offline, labels=options.labels)
      File "/Users/lpsinger/local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 619, in createEvent
        return self.post(uri, fields, files=files)
      File "/Users/lpsinger/local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 381, in post
        return self.post_or_put("POST", *args, **kwargs)
      File "/Users/lpsinger/local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 399, in post_or_put
        content_type, body = encode_multipart_formdata(body, files)
      File "/Users/lpsinger/local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 1285, in encode_multipart_formdata
        body = CRLF.join(L)
    TypeError: sequence item 20: expected str instance, bytes found