Add shell.nix for nix users

shell.nix

@@ -0,0 +1,19 @@
+{pkgs ? import <nixpkgs> {}}: let
+  python = pkgs.python312.withPackages (ps:
+    with ps; [
+    rebulk
+    babelfish
+    python-dateutil
+    pytest
+    pytest-mock
+    pytest-benchmark
+    pytest-cov
+    pylint
+    pyyaml
+    ]);
+in
+  pkgs.mkShell {
+    packages = [
+      python
+    ];
+  }