greg.tests: fix import grouping

Fixes: GREG-16

greg/tests/test_api_person.py

 from django.contrib.auth import get_user_model
+from django.test import TestCase
 from django.utils.dateparse import parse_datetime
 from rest_framework import status
+from rest_framework.authtoken.models import Token
 from rest_framework.reverse import reverse
 from rest_framework.test import APIClient, APITestCase
-from rest_framework.authtoken.models import Token
-
-from django.test import TestCase
 
 from greg.models import Person, PersonRole, Role