Readd graphviz installation

Author: tekktrik

.github/workflows/build.yml

@@ -45,6 +45,9 @@ jobs:
     - name: Pip install Sphinx, pre-commit
       run: |
         pip install --force-reinstall Sphinx sphinx-rtd-theme pre-commit
+    - name: Load graphviz
+      run: |
+        apt install graphviz
     - name: Library version
       run: git describe --dirty --always --tags
     - name: Setup problem matchers