ci: Run zulip backend test suite for Debian bullseye.

This also verifies the Zulip codebase's Python 3.9 support.

.github/workflows/zulip-ci.yml

@@ -34,6 +34,15 @@ jobs:
             is_focal: true
             include_frontend_tests: false
 
+          # This docker image was created by a generated Dockerfile at:
+          #   tools/ci/images/focal/Dockerfile
+          # Bullseye ships with Python 3.9.2.
+          - docker_image: zulip/ci:bullseye
+            name: Debian 11 Bullseye (Python 3.9, backend)
+            os: bullseye
+            is_bullseye: true
+            include_frontend_tests: false
+
     runs-on: ubuntu-latest
     name: ${{ matrix.name }}
     container: ${{ matrix.docker_image }}
@@ -231,7 +240,7 @@ jobs:
           retention-days: 60
 
       - name: Check development database build
-        if: ${{ matrix.is_focal }}
+        if: ${{ matrix.is_focal || matrix.is_bullseye }}
         run: ./tools/ci/setup-backend
 
       - name: Report status