Nix Flake for non-FHS venv

flake.nix

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };
  outputs = inputs @ {
    self,
    nixpkgs,
    flake-utils,
    ...
  }:
    flake-utils.lib.eachDefaultSystem (system: let
      pkgs = import nixpkgs {inherit system;};
    in {
      devShell = pkgs.mkShell {
        buildInputs = with pkgs; [
          python3
          python3Packages.pip
          python3Packages.virtualenv
          pythonManylinuxPackages.manylinux2014Package
          cmake
          ninja
	  imagemagick
        ];
	shellHook = ''
          export LD_LIBRARY_PATH="${pkgs.stdenv.cc.cc.lib.outPath}/lib:${pkgs.pythonManylinuxPackages.manylinux2014Package}/lib:$LD_LIBRARY_PATH";
          test -d .nix-venv || ${pkgs.python3.interpreter} -m venv .nix-venv
          source .nix-venv/bin/activate
    '';
 };   });
}

And to clarify for newbies, you run this with:

nix develop

See also Jupyter and Python on NixOS on r/NixOS for more context.