[PATCH] compare_strings.py : Add --gmean flag

[Florian Weimer]

* Noah Goldstein via Libc-alpha:

> Is there an issue using statistics package and just doing:
> `statistics.geometric_mean`
> ?

According to

  <https://docs.python.org/3/library/statistics.html#statistics.geometric_mean>

it was added in Python 3.8, which is newer than what we aim for in
general (INSTALL currently mentions Python 3.4).

Thanks,
Florian