CI: run linter on py310

which is the default/system python3 on ubuntu 22.04.

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: 3.8
+        python-version: '3.10'
     - name: Lint with flake8
       run: |
         pip install flake8-pyproject flake8