Sage assumes that pari does not use threads. However instead of
disabling them at compile time, we can diable them at runtime instead.
As a result, sage doesn't need its own version of pari anymore. That
means less rebuilds, less complexity and easier debugging.
Removes the version pinning for arb and pynac by backporting the
upstream (sage) package upgrades. This necessitates a new patch for arb,
which was however already proposed and accepted upstream.