add custom sphinx

Author: kp992

.github/workflows/cache.yml

@@ -23,6 +23,9 @@ jobs:
       - name: graphviz Support # TODO: required?
         run: |
           sudo apt-get -qq update && sudo apt-get install -y graphviz
+      - name: Install sphinx from branch
+        run: |
+          pip install git+https://github.com/kp992/sphinx.git@testcache
       - name: Install latex dependencies
         run: |
           sudo apt-get -qq update