Nix: building with pypy fails

[hacklschorsch]

Our Nix flake (based on nixpkgs 24.11) works with CPython (at least) 3.10, 3.11, and 3.12.

But it currently does not work with PyPy?, not because our software wouldn't work with it, but (seemingly) because dependency package configuration issues.

This is so similar to #3991, its description fits here well too:

The Nix package pulls in some unnecessary dependencies that also happen to be incompatible with (ie, broken for) these two Nixpkgs Python package.

There's quite a few of these but the fix for each is generally quite simple - it just involves breaking an optional dependency somewhere. None of these are optional dependencies Tahoe-LAFS actually requires, they're just what you get by default from the Nixpkgs Python package set.