I just had this same problem, and this code does the trick.
import boto3
s3 = boto3.resource("s3")
s3_bucket = s3.Bucket("bucket-name")
dir = "dir-in-bucket"
files_in_s3 = [f.key.split(dir + "/")[1] for f in
s3_bucket.objects.filter(Prefix=dir).all()]