tests/test-gendoc-it.t

py3: don't use dict.iterkeys()

Instead of iterating of dict.iterkeys(), let's iterate of dict as that will be
equivalent and dict.iterkeys() is not present in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D1796

#require docutils gettext

  $ $TESTDIR/check-gendoc it
  checking for parse errors