Add backports for python3-aiosmtpd

build.sh

@@ -19,7 +19,9 @@ die() {
 
 set -x
 
-# Install packages.
+# Install packages (aiosmtpd comes from backports in stretch).
+echo "deb http://deb.debian.org/debian stretch-backports main" \
+    > /etc/apt/sources.list.d/backports.list
 apt-get -q update
 install_packages ${PACKAGES} \
     || die "could not install packages"