pulp_rpm 3.18 is not compatible with pulpcore < 3.21

[fao89]

pulpcore 3.17:

   File "/usr/local/lib/python3.8/site-packages/pulp_rpm/app/urls.py", line 6, in <module>
      V3_API_ROOT = settings.V3_API_ROOT_NO_FRONT_SLASH
    File "/usr/local/lib/python3.8/site-packages/dynaconf/base.py", line 136, in __getattr__
      value = getattr(self._wrapped, name)
    File "/usr/local/lib/python3.8/site-packages/dynaconf/base.py", line 298, in __getattribute__
      return super().__getattribute__(name)
  AttributeError: 'Settings' object has no attribute 'V3_API_ROOT_NO_FRONT_SLASH'

https://github.com/pulp/pulp-oci-images/actions/runs/3042171905/jobs/4904379801#step:7:1651

pulpcore 3.18, 3.19 and 3.20

File "/usr/local/lib/python3.8/site-packages/pulp_rpm/app/modelresource.py", line 7, in <module>
    from pulpcore.plugin.modelresources import RepositoryResource
ModuleNotFoundError: No module named 'pulpcore.plugin.modelresources'

https://github.com/pulp/pulp-oci-images/actions/runs/3042171918/jobs/4904376202#step:7:1645
https://github.com/pulp/pulp-oci-images/actions/runs/3042171962/jobs/4900057732#step:7:1686
https://github.com/pulp/pulp-oci-images/actions/runs/3042172006/jobs/4904378328#step:9:1653