util: implement sortdict.insert() As flagged by pytype (reported via Matt Harbison, thanks). This was broken by bd0fd3ff9916 (util: rewrite sortdict using Python 2.7's OrderedDict, 2017-05-16). We actually call insert() on namespaces.py:100, but we clearly don't have test coverage of that an no users have reported it AFAIK. Differential Revision: https://phab.mercurial-scm.org/D7680