conftest.py: Don't use deprecated get_marker() function

Starting from pytest version 4.1.0, Node.get_marker() has been removed, and hence our tests break when running with newer versions of pytest. It was deprecated since a while back but it has recently been removed completely. Use get_closest_marker() as a replacement that is suggested in the changelog, and seems to work fine for our use case.

See https://github.com/pytest-dev/pytest/pull/4564 for more context on the upstream issue.

One way of verifying this change is that this should fix the recently added tests-fedora-update-deps job, that was failing before due to this issue.