greg.tests: make PersonTestData inherit from GregAPITestCase

PersonTestData.setUp() calls super().setUp(), but this is only
available on GregAPITestCase.  This only works at runtime because
PersonTestData and GregAPITestCase is composed together with
multi-inheritance as part of PersonAPITestCase.

This change is obviously not ideal, but we will change it soon when
introducing pytest anyway.  This is just to get the lints passing.

Fixes: GREG-16

greg/tests/test_api_person.py

@@ -22,7 +22,7 @@ class GregAPITestCase(APITestCase):
         return client
 
 
-class PersonTestData:
+class PersonTestData(GregAPITestCase):
     def setUp(self):
         super().setUp()
         self.person_foo_data = dict(
@@ -43,7 +43,7 @@ class PersonTestData:
         self.person_bar = Person.objects.create(**self.person_bar_data)
 
 
-class PersonAPITestCase(PersonTestData, GregAPITestCase):
+class PersonAPITestCase(PersonTestData):
     def test_get_person(self):
         url = reverse("person-detail", kwargs={"id": self.person_foo.id})
         response = self.client.get(url)