Add nix shell for dev stuff

Signed-off-by: William Casarin <jb55@jb55.com>

.envrc

@@ -1,3 +1,5 @@
+use nix
+
 export TODO_FILE=$PWD/TODO
 
 todo.sh ls || :

shell.nix

@@ -0,0 +1,5 @@
+{ pkgs ? import <nixpkgs> {} }:
+with pkgs;
+mkShell {
+  buildInputs = with python3Packages; [ Mako requests ];
+}