o Fix support for conditional list append/prepend in project.conf,
Merge request !1857
o Fix internal imports to import from "collections" instead
of "collections.abc", this improves support for Python 3.8,
see issue #831
o Fix some downloads from gitlab.com by setting custom user agent,
fixes issue #1285
o Work around python API break from ostree's repo.remote_gpg_import(),
this was changed in ostree commit v2019.2-10-gaa5df899, and we now
have a fallback to support both versions of the API, see merge request !1917.
