If somebody interested, I also found that “ghp_import.py” has some
Python 2 code only.  It doesn't make a big deal and “ghp-import” works
just fine.  But I made a snippet to fix this.  I'm not sure how to deal
with “python2-ghp-import”, because he doesn't need the snippet.

(modules '((guix build utils)))
(snippet ; Python 3 renamed the unicode type to str.
        '(substitute* "ghp_import.py"
           (("isinstance\\(text, unicode\\)")
            "isinstance(text, str)")))