bump CI python version (#7)

Author: st--

.github/workflows/ci_build.yaml

@@ -9,6 +9,6 @@ jobs:
     - uses: actions/checkout@v2
     - uses: actions/setup-python@v2
       with:
-        python-version: 3.7
+        python-version: 3.9
     - run: pip install tox
     - run: tox -e tests