add experimental shell.nix for cli venv

This allows the automatic creation and registration of a virtualenv when
running `nix-shell` within the `cli` directory, mimicking the
environment you would get from creating the virtual environment in the
usual way.

Note that the current shell does not work well with software like
`lorri`, supposedly because the shell performs impure operations
(calling out to a server via `pip`).

cli/shell.nix

@@ -0,0 +1,20 @@
+{ nixpkgs ? import <nixpkgs> {}
+}:
+let
+  myPythonPackages = nixpkgs.python39Packages;
+in
+nixpkgs.mkShell {
+
+  buildInputs = with nixpkgs; [
+    git
+    myPythonPackages.python
+    myPythonPackages.venvShellHook
+  ];
+
+  postVenvCreation = ''
+    pip install -r ${./requirements.txt}
+  '';
+
+  venvDir = ".venv";
+
+}