Define custom nixpkgs as overlay of python dependency

[lfdominguez]

Hi, I have a django project (not mine) that as sub-dependency need the weasyprint 59.0 dependency. I can't use the pip one because this dependency need some patches for libs that is already included on nixpkgs pythonPackage.weasyprint, the problem is if I replace the provider to use the nixpkgs one then get this error:

> Mach-nix version: master
> Python: 3.9.9
> Cause: Requirements conflict: weasyprint (<SpecifierSet('~=59.0')>,)
> The requirements which caused the error:
>   weasyprint (<SpecifierSet('~=59.0')>,)
>
> The given requirements might contain package versions which are not yet part of the dependency DB
> currently used. The DB can be updated by specifying 'pypiDataRev' when importing mach-nix.
> For examples see: https://github.com/DavHau/mach-nix/blob/master/examples.md

I understand that this is because the nixpkgs is the 60.0 version, how can i override the dependency search by mach. Tried this:

{
  description = "Django Project";

  inputs = {
    flake-utils.url = "github:numtide/flake-utils";
    mach-nix.url = "github:DavHau/mach-nix";
  };

  outputs = { self, nixpkgs, flake-utils, mach-nix }:
    flake-utils.lib.eachDefaultSystem
      (system:
        let
          python-packages-override = self: super: {
            python39 = super.python39.override {
              packageOverrides = pyself: pysuper: {
                weasyprint = pysuper.weasyprint.overrideAttrs (old: {
                  version = "59.0";

                  src = pkgs.fetchPypi {
                    version = "59.0";
                    pname = "weasyprint";
                    hash = "sha256-VrmBIoARg1ew9jse/hgZngg0PUpWozk8HUdauHjOomo=";
                  };
                });
              };
            };
          };

          pkgs = import nixpkgs {
            inherit system;

            overlays = [
              python-packages-override
            ];
          };

          python-package = mach-nix.lib."${system}".mkPython {
            requirements = builtins.readFile ./requirements-test.txt;

            providers.weasyprint = "nixpkgs";
          };
        in
        with pkgs;
        {
            devShells.default = mkShell {
              nativeBuildInputs = [ python-package ];
	        };
        }
    );
}

But same error.