use getattr in jinja2.filters.make_attrgetter instead of getitem

IMO, jinja2.filters.make_attrgetter should use environment.getattr instead of environment.getitem, which tries Python's getattr first and then falls back to attempting to treat the object as a dict/list.

For context, we ran into an issue when trying to use the sort filter's attribute argument on a list of security-proxied objects; trying to use index notation on them raises an exception that is not caught in environment.getitem.

I've tested this change minor change locally and all of the tests pass.