Update codecov.yml

Install requirements before running test coverage.

.github/workflows/codecov.yml

@@ -18,6 +18,7 @@ jobs:
         run: |
           pip install pytest
           pip install pytest-cov
+          if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
           pytest --cov=./ --cov-report=xml
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v1