Martin von Zweigbergk <martinvonz@google.com> [Sat, 13 Jul 2019 00:17:03 -0700] rev 4743
py3: call branchmap.items() on py3 and continue to call iteritems() on py2
Mercurial's source transformer also replaces the 'def iteritems(' in
branchmap by 'def items(', so we need to call whichever version is
there.