Added Python versions 3.7 and 3.8 for static-checking (#519)

Author: daikikatsuragawa

.github/workflows/static-checking.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6]
+        python-version: [3.6, 3.7, 3.8]
 
     steps:
       - uses: actions/checkout@v2