update duo pin file

NEWS

 unreleased (207x-xx-xx)
 
+    [duo] Change apt pin file to point to codename rather than archive
+    name. [adamhl]
+
     [out_of_date] Add some parameters to the interface to make it easier
     to point at different out-of-date servers. [adamhl]
 

templates/duo/duo.erb

@@ -5,17 +5,17 @@
 # in main don't have that, so we need to use the backported package we build.
 
 Package: libduo-dev
-Pin: release a=<%= lsbdistcodename %>-backports
+Pin: release n=<%= lsbdistcodename %>-backports
 Pin-Priority: 995
 
 Package: libduo3
-Pin: release a=<%= lsbdistcodename %>-backports
+Pin: release n=<%= lsbdistcodename %>-backports
 Pin-Priority: 995
 
 Package: libpam-duo
-Pin: release a=<%= lsbdistcodename %>-backports
+Pin: release n=<%= lsbdistcodename %>-backports
 Pin-Priority: 995
 
 Package: login-duo
-Pin: release a=<%= lsbdistcodename %>-backports
+Pin: release n=<%= lsbdistcodename %>-backports
 Pin-Priority: 995