deb822: Make gpg info available for release files.

lib/debian/deb822.py

@@ -2236,7 +2236,7 @@ class PdiffIndex(_multivalued):
         return max(lengths)
 
 
-class Release(_multivalued):
+class Release(_gpg_multivalued):
     """Represents a Release file
 
     Set the size_field_behavior attribute to "dak" to make the size field