Make population_sources note mutable [RHELDST-20307]

For enbaling extra content (for testing of UBI repos)
to go to the repository, this change enables to update
population_sources repo note.

src/pubtools/pulplib/_impl/model/repository/yum.py

@@ -92,6 +92,7 @@ class YumRepository(Repository):
         type=list,
         converter=frozenlist,
         pulp_field="notes.population_sources",
+        mutable=True,
     )
     """List of repository IDs used to populate this repository
     """