I would lean towards using the boto3 package to get that info. I would assume it's the same as you would get through the metadata, hopefully. There are a couple of ways to do this, but they all pretty much entail getting a "last_modified" attribute.
import boto3
s3 = boto3.client('s3', region_name='your_region')
objects = s3.list_objects(Bucket='your_bucket')
for o in objects["Contents"]:
print(o["LastModified"])