Give scheduletask fixture a non-naive datetime to squelch warning

greg/tests/models/test_scheduletask.py

@@ -5,7 +5,7 @@ from greg.models import ScheduleTask
 
 @pytest.fixture
 def scheduletask():
-    ScheduleTask.objects.create(name="foo", last_completed="2020-10-15 23:04")
+    ScheduleTask.objects.create(name="foo", last_completed="2020-10-15T23:04Z")
     return ScheduleTask.objects.get(id=1)