ci: use "apt_packages" to install graphviz for rtd

.readthedocs.yml

@@ -8,12 +8,12 @@ version: 2
 # Set the OS, Python version and other tools you might need
 build:
   os: ubuntu-22.04
+  apt_packages:
+    - graphviz
   tools:
     python: "3.11"
   jobs:
     post_create_environment:
-      # install graphviz
-      - apt-get install graphviz -y
       # Install poetry et.al.
       # https://python-poetry.org/docs/#installing-manually
       - pip install poetry jupyter-book sphinx-autoapi toml numpy pandas rustworkx scipy rich matplotlib tqdm ipywidgets graphviz packaging pydot pillow