docs: update supported Python versions in README

[ayushigithub12]

This pull request updates the "Supported Versions" section in README.md.

The documentation currently states: "Requests officially supports Python 3.9+."

However, Requests is tested and used on newer Python versions (3.10, 3.11, 3.12).
This update clarifies that the library supports Python 3.9+ and is currently tested up to Python 3.12.

This improves clarity for new users and keeps documentation aligned with actual CI environments.