CI: codecov steps fail for backend workflow (Python)

[flodolo]

Example: https://github.com/mozilla/pontoon/actions/runs/10961027567

warning - 2024-09-20 14:26:08,416 -- xcrun is not installed or can't be found.
warning - 2024-09-20 14:26:08,456 -- No gcov data found.
warning - 2024-09-20 14:26:08,494 -- No coverage data found to transform
info - 2024-09-20 14:26:08,546 -- Found 0 coverage files to report
Error: No coverage reports found. Please make sure you're generating reports successfully.
Warning: Codecov:
                        Failed to properly upload report: The process '/home/runner/work/_actions/codecov/codecov-action/v4/dist/codecov' failed with exit code 1

The reason is that running pytest alone doesn't generate a coverage report. #3360 seems to fix this, but it's unclear why 2 reports are generated.

Worth noting that make pytest creates a coverage report.
https://github.com/mozilla/pontoon/blob/main/Makefile#L97