Heads-up: pari 2.7 update in Rawhide

I'm about to build pari 2.7.1 in Rawhide, which includes an soname bump
and has API changes that will need fixes in many of the handful of
packages that use it. I've already been in discussion with the
maintainers regarding this, and it seems best to have the new version
available in Rawhide to work on.

To mitigate the disruption this will cause, I'm temporarily including
the old library in the package, so there won't be broken deps whilst the
migration process takes place.