update to python 3.12

[sntxrr]

This pull request updates the project to use Python 3.12 across all environments and makes minor adjustments to documentation and runtime configuration files to reflect this change. The updates ensure consistency and compatibility with the latest Python version.

Python Version Upgrade:

• .devcontainer/devcontainer.json: Updated the development container to use Python 3.12 instead of Python 3.9.
• Dockerfile: Changed the base image from python:3.9-slim to python:3.12-slim.
• app.yaml: Updated the runtime configuration to use python312 instead of python27.

Documentation Updates:

• CODESPACES.md: Updated the Codespaces environment description to reflect the use of Python 3.12 instead of Python 3.9.
• README.md: Modified the instructions for running the application locally to use python3 instead of Python.