Update Dockerfile-cpu to force python3.9

[glenn-jocher]

Update Dockerfile and Dockerfile-cpu. Use python3.9 commands with this image.

alias python=python3.9

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Upgraded requirements and Docker improvements for better compatibility and performance.

📊 Key Changes

• Lowered the minimum required version of scipy in requirements.txt to 1.4.1 for compatibility with Google Colab.
• Refined the Dockerfiles by specifying the exact PyTorch versions to be installed and reducing clutter.
• Improved the Docker CPU image by installing Python 3.9 instead of the default version and using python3.9 to install packages.
• Cleaned up redundant package uninstallation (torch, torchvision, torchtext) in Dockerfiles.

🎯 Purpose & Impact

• Ensuring that the required packages can be installed in Google Colab will help users quickly set up YOLOv5 in a widely used environment with free GPU access. 📘✅
• Specifying exact versions of PyTorch in Dockerfiles can lead to more predictable builds and avoid potential incompatibilities. 🔧📈
• Upgrading to Python 3.9 in Docker CPU improves performance and supports more recent Python features, which might enhance the development experience. 🚀💻
• Simplifying Dockerfile steps by removing unnecessary uninstalls reduces build time and potential errors during the image creation process. 🛠️🧹