Here’s a solution which actually streams the data line by line:
from io import TextIOWrapper
from gzip import GzipFile
...
# get StreamingBody from botocore.response
response = s3.get_object(Bucket=bucket, Key=key)
# if gzipped
gzipped = GzipFile(None, 'rb', fileobj=response['Body'])
data = TextIOWrapper(gzipped)
for line in data:
# process line