Make instructions work on sid

wiki/src/contribute/working_together/GitLab.mdwn

@@ -415,7 +415,13 @@ bin/generate-report]].
 
 To use them:
 
-* Install the `python3-gitlab/testing` Debian package.
+* Install a recent enough `python3-gitlab` Debian package:
+
+        if [ "$(lsb_release --short --codename)" = buster ]; then
+           sudo apt install python3-gitlab/testing
+        else
+           sudo apt install python3-gitlab
+        fi
 
 * Configure your `~/.python-gitlab.cfg`.