Add a script to initialize environment

When I get a clean repo (by cloning or by git-clean(1)), I do not want
to think about the usual commands to type, so I offload them here :-)

init

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+set -exu
+
+python3 -m venv env
+. env/bin/activate
+pip install -r requirements.txt