main/rdiff-backup: get version with importlib-metadata
If py3-setuptools is not installed 'rdiff-backup --version' will just report DEV, this adds py3-importlib-metadata as an option to installing py3-setuptools.
If py3-setuptools is not installed 'rdiff-backup --version' will just report DEV, this adds py3-importlib-metadata as an option to installing py3-setuptools.