Fix mypy error for setting datetime as span attribute

#2

app/s3/s3_resource.py

@@ -50,5 +50,7 @@ async def get_s3_bucket_object(s3: S3ServiceResource, bucket_name: str, key: str
             await obj.load()
         except ClientError:
             return None
-        span.set_attributes({"size": await obj.size, "last_modified": await obj.last_modified, "etag": await obj.e_tag})
+        span.set_attributes(
+            {"size": await obj.size, "last_modified": (await obj.last_modified).isoformat(), "etag": await obj.e_tag}
+        )
         return obj