fix: Use mock_aws instead of mock_s3

The moto@v5 do not implement mock_s3 anymore, it uses mock_aws instead.
This version is used since we do have a python3.9 project (under it uses a
moto@v4 in which mock_s3 is still present).

cubicweb_s3storage/testing.py

 import boto3
 from unittest.mock import patch
-from moto import mock_s3
+from moto import mock_aws
 
 
 class S3StorageTestMixin:
     s3_bucket = "test-bucket"
 
     def setUp(self):
@@ -4,10 +4,10 @@
 
 
 class S3StorageTestMixin:
     s3_bucket = "test-bucket"
 
     def setUp(self):
-        s3_mock = mock_s3()
+        s3_mock = mock_aws()
         s3_mock.start()
         resource = boto3.resource("s3", region_name="us-east-1")
         self.s3_bucket = resource.create_bucket(Bucket=self.s3_bucket)