ion matching `requirement` (e.g. via download)

        Obtain a distro that matches requirement (e.g. via download).  In the
        base ``Environment`` class, this routine just returns
        ``installer(requirement)``, unless `installer` is None, in which case
        None is returned instead.  This method is a hook that allows subclasses
        to attempt other ways of obtaining a distribution before falling back
        to the `installer` argument.Nr