Change build from 3.7, 3.8 to 3.8, 3.9

Author: kroo

.github/workflows/build.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.7", "3.8"]
+        python-version: ["3.8", "3.9"]
 
     steps:
     - uses: actions/checkout@v2