switch nix recipe to flake

flake.nix

@@ -0,0 +1,47 @@
+{
+  description = "Python environment for fipy";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
+    utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, utils}: (utils.lib.eachSystem ["x86_64-linux" ] (system:
+    let
+      pkgs = nixpkgs.legacyPackages.${system};
+      pypkgs = pkgs.python310Packages;
+
+      env = (pypkgs.fipy.overridePythonAttrs (old: rec {
+
+        src = pkgs.lib.cleanSource ./.;
+
+        nativeBuildInputs = with pypkgs; [
+          pip
+          pkgs.openssh
+          nbval
+          ipython
+          ipykernel
+          jupyterlab
+          traitlets
+          notebook
+        ] ++ propagatedBuildInputs;
+
+        propagatedBuildInputs = old.propagatedBuildInputs;
+
+        postShellHook = ''
+          SOURCE_DATE_EPOCH=$(date +%s)
+          export PYTHONUSERBASE=$PWD/.local
+          export USER_SITE=`python -c "import site; print(site.USER_SITE)"`
+          export PYTHONPATH=$PYTHONPATH:$USER_SITE:$(pwd)
+          export PATH=$PATH:$PYTHONUSERBASE/bin
+
+          export OMPI_MCA_plm_rsh_agent=${pkgs.openssh}/bin/ssh
+
+        '';
+      }));
+   in
+     rec {
+       devShells.default = env;
+     }
+  ));
+}