conv_gen: Use python2 to execute the script

The script does not work with python3:

$ python3 utils/conv_gen.py
  File "utils/conv_gen.py", line 124
    def _print_term(self, fi, num_states, pack = False):

Second there is no 'python' on FreeBSD and one needs to select
the major version to use.

  GEN      conv_cs3_gen.c
  GEN      conv_xcch_gen.c
  GEN      conv_cs2_gen.c
python: not found
python: not found
python: not found

By using python2 we solve both issues. On Debian python2 is located
inside the python-minimal package.

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