systemd-python: fix iteration
systemd-python: fix iteration

Back in 6a58bf4135 raising stop iteration was removed from the C
code, but wasn't added in the Python counterpart.

Awesense/python-systemd,Awesense/python-systemd,keszybz/python-systemd,again4you/python-systemd,systemd/python-systemd,again4you/python-systemd,Awesense/python-systemd,systemd/python-systemd,keszybz/python-systemd