Designate doesn't work with dnspython 2.0.0

[Chris MacNaughton]

Ubuntu Groovy is shipping with dnspython==2.0.0 and there are, at the very least, unit test failures with dnspython>=2, for example:

designate.tests.unit.agent.backends.test_denominator.DenominatorAgentBackendTestCase.test_create_zone
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Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/usr/lib/python3.8/unittest/mock.py", line 1325, in patched
    return func(*newargs, **newkeywargs)

      File "/home/ubuntu/packaging/pkg/designate/designate/tests/unit/agent/backends/test_denominator.py", line 63, in test_create_zone
    self.backend.create_zone(zone)

      File "/home/ubuntu/packaging/pkg/designate/designate/backend/agent_backend/impl_denominator.py", line 130, in create_zone
    rname = soa_record.items[0].rname.derelativize(origin=zone.origin)

    KeyError: 0