python3-dateutil: Removal of python3-six as a dependency

python3-dateutil: Removal of python3-six as a dependency

- python3-dateutil is installed as a python3 module.
- It had python3-six defined as a dependency. Python3-six is a module that
   allows a project to be capable of neing run under python2 or under
   python3
- With the planned removal of python2 there is no need to have
   python3-dateutil capable of working with python2.
- python3-six addon is being removed as there is no need for any python3
   module in IPFire to also be capable o0f running under python2

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>