gitlab-ci: Pull debian:unstable, not ubuntu:focal

Also remove the APT PPA

.gitlab-ci.yml

-image: ubuntu:focal
+image: debian:unstable
 variables:
   DEBIAN_FRONTEND: noninteractive
 
@@ -18,9 +18,6 @@ pep8:
 test:
   script:
   - apt update
-  - apt install -q -y software-properties-common
-  - add-apt-repository ppa:deity/sid
-  - apt update
   - apt build-dep -q -y ./
   - dpkg-buildpackage
   artifacts: