Update sage to 8.8

This pull request is motivated by #932367 caused by a GAP version mismatch. I figured that instead of backporting a change related to that to Sage 8.6, I might as well have a stab at getting 8.8 up and running.

I'm still seeing 33 doctest failures on amd64 Testing (with python-ppl built from salsa). Number might be slightly off because I only re-ran failed tests, full build of everything is still pending. Despite these errors, I see the new version as a clear improvement over the current state of 8.6 which won't even start for me. I also see it as a suitable basis for further collaborative work.