Fix debian install.

.github/workflows/pythonpackage.yml

@@ -22,9 +22,14 @@ jobs:
       run: |
         python -m pip install --upgrade pip flake8 cython
         python setup.py develop
+    - name: Install Debian-specific dependencies
+      run: |
+        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
         brz branch lp:brz-debian ~/.config/breezy/plugins/debian
+      if: "matrix.os == 'ubuntu-latest'"
     - name: Style checks
       run: |
         python -m flake8