help: get helptext/ data from `resources` module if available For PyOxidizer, we need to read configs using the `resources` module. This patch makes it so we use that module if available (i.e. Python >= 3.7). It does that by adding a new `open_resource()` function to our `resourceutil` module. Tested by running `$PYTHON ./hg help pager` for each $PYTHON in {python2, python3.6, python3.7}. Differential Revision: https://phab.mercurial-scm.org/D7622