use python3.9 for coverage

.github/workflows/pythoncoverage.yml

@@ -15,11 +15,11 @@ jobs:
         # os: [ubuntu-latest, macos-latest, windows-latest]
         os: [ubuntu-latest, ]
         # python-version: [3.7, 3.8]
-        python-version: [3.7, ]
+        python-version: [3.9, ]
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}