fix .github/workflows/unit-tests.yml

Author: fuku2014

.github/workflows/unit-tests.yml

@@ -27,7 +27,8 @@ jobs:
       - name: Check flake8
         run: pipenv run lint
   unittest:
-    runs-on: ubuntu-latest
+    # ubuntu-24.04 later does not support Python 3.7
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
         python-version: ["3.7", "3.8", "3.9", "3.10", "3.11"]