Set non-default keyservers

Try to fix keyserver issues. By using the new upstream default keyserver: https://salsa.debian.org/debian/gnupg2/blob/93daa0064ee29372d93661cc6a9aa990f8c52414/debian/NEWS for python

The google golang signing key hasn't been made available there though. So I've swapped that one to the ubuntu keysever, which does have it.