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