Introduce flake8 return

Based on !2281 (merged)

I've made one commit per rule. I'm not sure I agree with:

• RET502
• RET503.

Please tell me what you think, I can easily remove them.

arkindex/project/serializer_fields.py

@@ -304,4 +304,4 @@ class NullField(serializers.CharField):
     def to_internal_value(self, data):
         if data is not None:
             self.fail("invalid")
-        return None
+        return