Adapt distributions.conf defaults to Ubuntu keyrings of 2018

As of now, Debian's ubuntu-keyring package hasn't been updated for
more than two years. See https://bugs.debian.org/978438

debian/changelog

@@ -26,6 +26,8 @@ xen-tools (4.8.1~dev-1) UNRELEASED; urgency=medium
   * Bump debhelper compatibility level to 13.
     + Build-depend on "debhelper-compat (= 13)" to replace debian/compat.
   * Declare compliance with Debian Policy 4.5.1. (No changes required.)
+  * Adapt distributions.conf defaults to the fact that Debian Unstable
+    still ships Ubuntu keyrings as of 2018, see #978438.
 
  -- Axel Beckert <abe@debian.org>  Sat, 19 Dec 2020 08:02:50 +0100
 

etc/distributions.conf

@@ -53,11 +53,11 @@ wily         = ubuntu eol pygrub
 xenial       = ubuntu     pygrub
 yakkety      = ubuntu eol pygrub
 zesty        = ubuntu eol pygrub ubuntu-keyring-2012-archive.gpg
-artful       = ubuntu eol pygrub
+artful       = ubuntu eol pygrub ubuntu-archive-keyring.gpg
 bionic       = ubuntu     pygrub
-cosmic       = ubuntu eol pygrub
-disco        = ubuntu eol pygrub
-eoan         = ubuntu eol pygrub
+cosmic       = ubuntu eol pygrub ubuntu-archive-keyring.gpg
+disco        = ubuntu eol pygrub ubuntu-archive-keyring.gpg
+eoan         = ubuntu eol pygrub ubuntu-archive-keyring.gpg
 focal        = ubuntu     pygrub
 groovy       = ubuntu     pygrub
 hirsute      = ubuntu     pygrub