PyUpgrade 3.8 updates

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

This PR refines string formatting across multiple files to follow a more modern syntax.

📊 Key Changes

• Updated string formatting in classify/predict.py, detect.py, segment/predict.py, and utils/metrics.py from the old % syntax to the more modern str.format() and f-string syntax.

🎯 Purpose & Impact

• Improved Readability: Makes the code more readable and maintainable by using a consistent, modern string formatting method.
• Code Quality: Aligns with contemporary Python practices, helping developers avoid potential pitfalls of the older % string formatting method.

🥳 This small but effective update ensures that the code remains clean and up-to-date with current Python standards.