Run GitHub workflows on 3.7, 3.8, 3.9

Author: mesozoic

.github/workflows/continuous-integration.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: macos-latest
     strategy:
       matrix:
-        python-version: [3.8]
+        python-version: [3.7, 3.8, 3.9]
 
     env:
       COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}