An assortment of packaging fixes

• Unpin nixpkgs here. Consider ZKAPAuthorizer a library and let the integrator pin a version. This is probably not the final word on how things should work but it helps for now.
• Downgrade the Python module typing to a version compatible with Hypothesis.
• Bump to a version of LeastAuthority/privacypass that doesn't require us to use Git submodules (which introduce an extra place for weird confusion in the packaging)
• Bump to the latest Tahoe-LAFS integration branch revision and add setuptools as an explicit runtime dependency (because it is one now)

nixpkgs.nix

 { pkgs ? import <nixpkgs> { } }:
 let
-  nixpkgs = fetchTarball
-  { url = "https://github.com/NixOS/nixpkgs-channels/archive/4557b9f1f50aa813ae673fe6fcd30ca872968947.tar.gz";
-    sha256 = "0cam48cn042axcik9vqxsqjc2hwyb2grjbjxacsn4w0y1zk6k6l2";
-  };
+  nixpkgs = pkgs.path;
   args =
   { overlays = [ (import ./overlays.nix) ];
   };