Not compatible with Python 3

[villelaitila]

Using six and some changes it could made compatible with both Python 2 and Python 3.

When running rfgen in Python 3.7:

 ...
  File "/usr/local/bin/rfgen.py", line 467
    %d resources per resource file (option 'r')""" % (testlibs_count, keyword_count, testsuite_count,
                                                 ^
SyntaxError: invalid syntax

[pekkaklarck]

This project has been pretty much abandoned and I've personally never used it. If there are users, making it Python 3 compatible is definitely a good idea. Are you interested to provide a PR? Using six and/or future is fine, but I think it would be OK also to make this Python 3(.6) only.