GitHub API rate limit fix

[glenn-jocher]

This PR updates attempt_download() in google_utils.py to only query the GitHub API for missing files rather than in all cases (currently), fixing issue #1890.

All functions in this file have also been updated with Path objects now.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancement in the GitHub and Google Drive download functions within the Ultralytics YOLOv5 codebase.

📊 Key Changes

• Refactored gsutil_getsize function to use an f-string for string interpolation.
• Simplified attempt_download function for clarity and removed redundant code.
• Replaced string concatenation with f-strings across the file for consistency and readability.
• Changed exception handling to offer more detailed error messages and clean up of partial downloads.
• Modified gdrive_download to use Path objects for file manipulations and unlink method for removing files with an option to suppress FileNotFoundError if the file does not exist.

🎯 Purpose & Impact

• 🚀 Improved code readability and maintainability by utilizing modern Python string formatting.
• 📁 Streamlined file download processes, potentially enhancing the user experience when downloading YOLOv5 assets.
• 💥 More informative error handling could help users and developers troubleshoot download issues quicker.
• 🧼 Clean-up code ensures no partial or corrupted downloads are left, saving disk space and reducing confusion.