templater: teach template loader to use open_template() function
The template loader can apparently load templates from relative paths,
so I needed to add that support to `open_template()`.
This takes the number of failing tests with PyOxidizer from 54 to 34.
Differential Revision: https://phab.mercurial-scm.org/D8907
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors