Update tests.yml

Author: daikikatsuragawa

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
       fail-fast: false 
       matrix:
         OS: ['ubuntu-latest', 'windows-latest']
-        PYTHON_VERSION: ['3.5', '3.6', '3.7', '3.8']
+        PYTHON_VERSION: ['3.5', '3.6', '3.7', '3.8', '3.9']
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python