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/tasks.py

@@ -21,7 +21,7 @@ from arkindex.documents.models import (
     Transcription,
     TranscriptionEntity,
 )
-from arkindex.ponos.models import Task
+from arkindex.ponos.models import Artifact, Task
 from arkindex.process.models import Process, ProcessDatasetSet, ProcessElement, WorkerActivity, WorkerRun
 from arkindex.training.models import DatasetElement, DatasetSet
 from arkindex.users.models import User
@@ -41,6 +41,10 @@ def corpus_delete(corpus_id: str) -> None:
         corpus.top_level_type_id = None
         corpus.save(update_fields=["top_level_type_id"])
 
+    # Set process.element to NULL in all processes, to avoid IntegrityErrors when deleting elements, as
+    # elements are deleted before processes.
+    Process.objects.filter(corpus_id=corpus_id).exclude(element_id=None).update(element_id=None)
+
     # Delete all related objects, bypassing RESTRICTs deletion rules
     # and Django's way of loading everything into memory before deleting.
     querysets = [
@@ -67,6 +71,8 @@ def corpus_delete(corpus_id: str) -> None:
         Selection.objects.filter(element__corpus_id=corpus_id),
         corpus.memberships.all(),
         corpus.exports.all(),
+        # Delete task artifacts
+        Artifact.objects.filter(task__process__corpus_id=corpus_id),
         # ProcessDatasetSet M2M
         ProcessDatasetSet.objects.filter(set__dataset__corpus_id=corpus_id),
         ProcessDatasetSet.objects.filter(process__corpus_id=corpus_id),