gnu: duplicity: Update to 0.8.14.

* gnu/packages/backup.scm (duplicity): Update to 0.8.14.
[native-inputs]: Add gettext, python-future, python-pytest,
python-pytest-runner, and python-setuptools-scm.  Move other python2 packages
to python.
[propagated-inputs]: Ditto.
[inputs]: Use latest librsync.
[arguments]: Remove #:python argument.  Adjust gpg substitution regex for
latest source.