systemd-python: allow threads around flush
systemd-python: allow threads around flush

flush() is potentially costly.

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