templater: don't normalize path separators to '/' when interacting with OS
`_readmapfile()` is about reading a map file from the file system, so
we shouldn't use our `util.normpath()`, which also normalizes `os.sep`
to '/'.
Differential Revision: https://phab.mercurial-scm.org/D8806
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors