Calibre - split RPi from others to use raspbain's testing repo, split off unstable

scripts/calibre-install-latest-rpi.sh

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+#!/bin/bash
+
+# Thanks to Jerry Vonau (https://github.com/jvonau) who made
+# this critical breakthrough (Calibre 3.x on Raspbian) possible!
+# The latest available is 3.20 available from testing
+# https://packages.debian.org/search?keywords=calibre
+# (SEE http://archive.raspbian.org/raspbian/pool/main/c/calibre/ ?)
+# might break future updates, you have been warned
+
+export DEBIAN_FRONTEND=noninteractive
+# Updates calibre calibre-bin to version 3.20 or ... from testing
+echo "deb http://raspbian.raspberrypi.org/raspbian/ testing main" > /etc/apt/sources.list.d/rpi-testing.list
+apt update
+apt -y install calibre calibre-bin
+# Remove last line, safer than: rm /etc/apt/sources.list.d/debian-testing.list
+sed -i '$ d' /etc/apt/sources.list.d/rpi-testing.list
+# Clears the cache of testing
+apt update