cleanup: hgdemandimport.tracing accepts strings, not bytes
This does mean that the resulting traces will have some b'' goo in them on
Python 3, but I think that's a worthwile price to pay since it doesn't break
any tests.
Differential Revision: https://phab.mercurial-scm.org/D7108
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors