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