Support ghc 9.2 and recent nixpkgs

[avh4]

This includes https://github.com/owickstrom/gi-gtk-declarative/pull/107 and does the following:

• compilation with ghc 9.2.* now succeeds https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.2#undecidableinstances-no-longer-implies-flexiblecontexts-in-instance-declarations
• nixpkgs is updated to a recent version
• docs build is updated to use mach-nix, as recommended by https://nixos.wiki/wiki/Python, as pypi2nix is broken and is no longer maintained

[cblp]

I'd love to see it on Hackage