Update Python version for CI tests

Author: jvamvas

.github/workflows/unittest.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.7"]
+        python-version: ["3.9"]
 
     steps:
       - uses: actions/checkout@v3
@@ -31,4 +31,4 @@ jobs:
           # stop the build if there are Python syntax errors or undefined names
           flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics --exclude experiments
       - name: Test
-        run: python -m unittest discover -s tests
+        run: python -m unittest discover -s tests