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.