Include pixelmatch dependency

Author: hansthen

.github/workflows/test_code.yml

@@ -35,5 +35,10 @@ jobs:
     - name: Install folium from source
       run: python -m pip install -e . --no-deps --force-reinstall
 
+    - name: Install pixelmatch
+      shell: bash -l {0}
+      run: |
+        pip install pixelmatch
+
     - name: Code tests
       run: python -m pytest -vv --ignore=tests/selenium --ignore=tests/playwright