Use pip to get fpylll

[lducas]

I was having trouble with the bootstrap.sh script on a new OS (Ubuntu 24.04.1 LTS). First I had to add setuptools in the pip requirements, and still, after compilation it would fail to find fpylll as a python module.

But we now have fpylll available via pip, and the developpment of fpylll being not so active right now I thought maybe it could be a more sable way to source it. It solved my issue.

[cr-marcstevens]

Just a thought: Simplifying is great, but what if pip is behind on a breaking change? Say e.g. during active development.

Perhaps use the pip route by default, but keep the old code still an option?