CI: Install python3-dev, librairo2-dev, and pkg-config before building docs

Author: clonker

.circleci/config.yml

@@ -1235,6 +1235,11 @@ jobs:
     steps:
       - checkout
       - setup_prerelease_commit_hash
+      - run:
+          name: Install build system dependencies
+          command: |
+            apt-get update
+            apt-get install --quiet=2 --no-install-recommends python3-dev libcairo2-dev pkg-config
       - run:
           name: Build documentation
           command: docs/docs.sh