pkg-merge-upstream-to-downstreams: Python 3.7 (v2021) compatibility

Try to be compatible tiwh Python 3.7 as shipped in Buster so the tool
can work on projects where the default branch points to v2021, either
because it's been dropped from v2022 or because it's a downstream that
has not started mirroring v2022 yet.

      File "/usr/bin/apertis-pkg-merge-upstream-to-downstreams", line 65, in ensure_downstream_branch
        project_id = urllib.parse.quote(url.path.strip("/").removesuffix(".git"), safe="")
    AttributeError: 'str' object has no attribute 'removesuffix'

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>