add pytest-cov to the list of github actions' dependencies

Author: astanin

.github/workflows/tabulate.yml

@@ -23,7 +23,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        python -m pip install pytest numpy pandas "wcwidth>=0.6.0"
+        python -m pip install pytest pytest-cov numpy pandas "wcwidth>=0.6.0"
     - name: Run tests
       run: |
         pytest -v --doctest-modules --ignore benchmark --cov=tabulate --cov-branch --cov-report=xml