GES.TrackElement.lookup_child used by overrides/GES.py

The following message can be seen in https://gitlab.gnome.org/aleb/pitivi/-/jobs/290465 when the Pitivi tests are run:

Installing /run/build/gst-editing-services/bindings/python/gi/overrides/GES.py to /app/lib/python3.7/site-packages/gi/overrides
...
/app/lib/python3.7/site-packages/gi/overrides/GES.py:73: DeprecationWarning: GES.TrackElement.lookup_child is deprecated