Allow to fill stretch-pu updates against release.debian.org
The entry to allow to fill stretch-pu (resp. oldstable-pu) updates was disabled back when jessie was moved from regular support to LTS. As of now both buster and stretch are supported. Thus re-enable the option to fill stretch-pu updates.
Closes: #938941
Signed-off-by: Salvatore Bonaccorso carnil@debian.org