[spec] Bump sphinx version to fix Python problem

Author: rossberg

.github/workflows/main.yml

@@ -63,7 +63,7 @@ jobs:
       - run: pip install bikeshed && bikeshed update
       - run: pip install six
       - run: sudo apt-get update -y && sudo apt-get install -y latexmk texlive-latex-recommended texlive-latex-extra texlive-fonts-recommended
-      - run: pip install sphinx==4.0.0
+      - run: pip install sphinx==5.1.0
       - run: cd document/core && make all
       - uses: actions/upload-artifact@v2
         with: