sources: skip librepo test if no librepo can be imported

The librepo test has a similar issue as the dnf tests, we need
to use the system packages, c.f.
https://github.com/osbuild/containers/pull/79

sources/test/test_librepo.py

@@ -1,7 +1,11 @@
 #!/usr/bin/python3
 from unittest.mock import patch
 
-import librepo
+try:
+    import librepo
+except ImportError:
+    import pytest
+    pytest.skip("need librepo to run this test", allow_module_level=True)
 import pytest
 
 from osbuild import testutil