Fix and test OSTree repo.remote_gpg_import() calls

The python facing API has changed in ostree commit v2019.2-10-gaa5df899.

We have since changed our invocation to it, but it is still failing where older ostree is installed, this went unnoticed because we don't test fetches of ostree sources using the gpg signing feature except for the integration tests, and those are only ever refetched when the gitlab cache gets cleaned up.

This branch changes the invocation so that both APIs are tried, staring with the new API and falling back to the old one in the case of a TypeError, and adds a self contained test to ensure we don't regress here again.