feat(nix): add nix package management

shell.nix

@@ -0,0 +1,16 @@
+# https://github.com/DavHau/mach-nix/blob/master/examples.md
+{ pkgs ? import <nixpkgs> {} }:
+let
+  mach-nix = import (
+    builtins.fetchGit {
+      url = "https://github.com/DavHau/mach-nix/";
+      ref = "refs/tags/3.3.0";
+    }
+  ) {
+    python = "python39";
+  };
+in
+
+mach-nix.mkPythonShell {
+  requirements = builtins.readFile ./requirements.txt;
+}