Test example code from readme in CI

To make sure that our example code actually works, this patch adds a CI
job that extracts the code and tries to run it.

.github/workflows/ci.yaml

@@ -127,3 +127,22 @@ jobs:
         run: ./venv/bin/pip install .
       - name: Run test suite
         run: ./venv/bin/python -m unittest -v
+  test-readme:
+    name: Run example code from readme
+    runs-on: ubuntu-latest
+    container: python:3.9-slim
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Install required packages
+        run: apt update && apt install -y ${REQUIRED_PACKAGES} cargo
+      - name: Checkout tangler
+        run: git clone https://github.com/wiktor-k/tangler.git --branch v0.3.0 --depth 1
+      - name: Install Poetry
+        run: pip install poetry
+      - name: Create virtual environment
+        run: make install
+      - name: Extract code from readme
+        run: cargo run --manifest-path tangler/Cargo.toml --locked python < README.md > readme.py
+      - name: Run code from readme
+        run: poetry run readme.py