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/documents/api/ml.py

@@ -440,7 +440,7 @@ class ManageClassificationsSelection(SelectionMixin, CorpusACLMixin, CreateAPIVi
         mode = serializer.validated_data["mode"]
         if mode == ClassificationMode.Create:
             return self.create(corpus, request, *args, **kwargs)
-        elif mode == ClassificationMode.Validate:
+        if mode == ClassificationMode.Validate:
             elements = self.get_selection(corpus.id)
             Classification.objects.filter(
                 element__in=elements,