Remove return type from get iam auth

Since botocore doesn't have typings by default, this will not work unless we settle on something less specific than S3 client or add typing stubs to dependencies. In my opinion it is not worth it for this small method

karton/core/backend.py

@@ -14,7 +14,6 @@
     InstanceMetadataFetcher,
     InstanceMetadataProvider,
 )
-from botocore.client import S3
 from botocore.session import get_session
 from redis import AuthenticationError, StrictRedis
 from redis.client import Pipeline
@@ -154,7 +153,7 @@ def __init__(
             aws_secret_access_key=secret_key,
         )
 
-    def iam_auth_s3(self, endpoint: str) -> Optional[S3]:
+    def iam_auth_s3(self, endpoint: str):
         boto_session = get_session()
         iam_providers = [
             ContainerProvider(),