Prevent CI from trying to use '--cov' when pytest-cov is not installed

Author: abravalheri

.github/workflows/main.yml

@@ -39,9 +39,12 @@ jobs:
         run: |
           python -m pip install tox
       - name: Run tests
-        run: tox -- --cov-report xml
+        run: tox
+      - name: Create coverage report
+        if: hashFiles('.coverage') != ''  # Rudimentary `file.exists()`
+        run: pipx run coverage xml --ignore-errors
       - name: Publish coverage
-        if: false  # disabled for #2727
+        if: hashFiles('coverage.xml') != ''  # Rudimentary `file.exists()`
         uses: codecov/codecov-action@v1
         with:
           flags: >-  # Mark which lines are covered by which envs