Pin DRF to < 3.10 due to breaking changes

We cannot fix the incompatibility yet because another dependency,
drf-yasg does not support drf 3.10 yet.
axnsan12/drf-yasg#410

https://pulp.plan.io/issues/5125
re #5125

setup.py

@@ -7,7 +7,7 @@
     'coreapi',
     'Django~=2.2',  # LTS version, switch only if we have a compelling reason to
     'django-filter',
-    'djangorestframework',
+    'djangorestframework<3.10',
     'djangorestframework-queryfields',
     'drf-nested-routers',
     'drf-yasg',