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
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..