ci: use Python 3 for coverage

.github/workflows/coverage.yml

@@ -7,14 +7,14 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 2.7
+    - name: Set up latest Python 3
       uses: actions/setup-python@v2
       with:
-        python-version: 2.7
+        python-version: 3
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install protobuf==3.17.3 pycodestyle
+        pip install protobuf pycodestyle coverage coveralls
     - name: Test with coverage
       run: |
         cd python