Install apt-pkg-dev.

.github/workflows/pythonpackage.yml

@@ -24,6 +24,7 @@ jobs:
           python setup.py develop
       - name: Install Debian-specific dependencies
         run: |
+          sudo apt install libapt-pkg-dev
           python -m pip install git+https://salsa.debian.org/apt-team/python-apt
           python -m pip install -e ".[debian]"
           mkdir -p ~/.config/breezy/plugins