Add testing for Sphinx 8

Remove pin to Sphinx <7.3

Author: sciencewhiz

.github/workflows/workflow.yml

@@ -69,7 +69,7 @@ jobs:
       fail-fast: false
       matrix:
         python-version: ['pypy3.9', '3.8', '3.9', '3.10', '3.11', '3.12']
-        sphinx-version: ['>=5,<6', '>=6a0,<7', '>=7,<7.3']
+        sphinx-version: ['>=5,<6', '>=6a0,<7', '>=7,<8', '>=8,<9']
         os: [windows-latest, macos-latest, ubuntu-latest]
     steps:
       - uses: actions/checkout@v4