mypy fix test

Author: AtsushiSakai

.github/workflows/pythonpackage.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.7, 3.8]
+        python-version: [3.7]
 
     steps:
     - uses: actions/checkout@v1