ci: point actions/setup-python at the setup script/config

By default, this action uses requirements.txt or pyproject.toml as
hash keys, which do not currently exist, resulting in an error.

.github/workflows/ci.yml

@@ -29,6 +29,7 @@ jobs:
         with:
           python-version: '3.11'
           cache: pip
+          cache-dependency-path: setup.*
       - run: python -m pip install .
 
   lint: