releasenotes: use the right API to access the 'sections'

Preventing direct access to the underlying dict fix a breakage introduced by the
refactoring in d3df397e7a59.

This changeset is similar to 271dfcb98544, 5272542196cc and f7621fa14b84. The
breackage of `releasenotes.py` stayed under my radar as the CI did not have
fuzzywuzzy installed. (Something that is about to be fixed).

Differential Revision: https://phab.mercurial-scm.org/D10121

hgext/releasenotes.py

@@ -280,7 +280,7 b' def getcustomadmonitions(repo):'
 
     if b'.hgreleasenotes' in ctx:
         read(b'.hgreleasenotes')
-    return p[b'sections']
+    return p.items(b'sections')
 
 
 def checkadmonitions(ui, repo, directives, revs):