Add GitHub Actions workflow for test coverage

[mikeqfu]

Set up a GitHub Actions workflow to run test coverage and upload the coverage report to Codecov. This will help track code coverage over time and ensure our tests are effectively covering the codebase.

• Set up the environment to run test coverage using pytest.
• Create a new GitHub Actions workflow (e.g. .github/workflows/test-coverage.yml), ensuring tests run successfully on Linux and handle any platform-specific issues.
• Verify that the workflow runs on relevant branches and PRs.
• Add a Codecov badge to the README.md for visibility.

Notes:

• PyHelpers is primarily developed on Windows, but the workflow should ideally be cross-platform.
• Some dependencies (e.g. 7-Zip, Pandoc, Inkscape, wkhtmltopdf, Wine, PostgreSQL, Microsoft SQL Server) may be required for certain tests - consider handling them in the workflow.