python-systemd: add new package

Add package for official python bindings availibe as a part of systemd
project at: https://github.com/systemd/python-systemd



Minimal config snippet for utils/test-pkg is as follows (sans
backslashes):

BR2_INIT_SYSTEMD=y
BR2_PACKAGE_PYTHON3=y
BR2_PACKAGE_PYTHON_SYSTEMD=y

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>