python3: use binary string for adler32 input
    
    Use empty binary string instead of empty native string for adler23 input.
    
        Could not execute build: 'str' does not support the buffer interface
        Traceback (most recent call last):
          File "/usr/lib/python-exec/python3.4/fedpkg", line 16, in <module>
            main()
          File "/usr/lib64/python3.4/site-packages/fedpkg/__main__.py", line 78, in main
            sys.exit(client.args.command())
          File "/usr/lib64/python3.4/site-packages/pyrpkg/cli.py", line 942, in build
            self.cmd.koji_upload(self.args.srpm, uniquepath, callback=callback)
          File "/usr/lib64/python3.4/site-packages/pyrpkg/__init__.py", line 2020, in koji_upload
            self.kojisession.uploadWrapper(file, path, callback=callback)
          File "/usr/lib64/python3.4/site-packages/koji/__init__.py", line 2363, in uploadWrapper
            self.fastUpload(localfile, path, name, callback, blocksize, overwrite)
          File "/usr/lib64/python3.4/site-packages/koji/__init__.py", line 2284, in fastUpload
            full_chksum = util.adler32_constructor()
          File "/usr/lib64/python3.4/site-packages/koji/util.py", line 449, in __init__
            self._value = adler32(arg) & 0xffffffff
        TypeError: 'str' does not support the buffer interface
    
    Signed-off-by: Pavel Šimerda <pavlix@pavlix.net>