chore: Drop Python 3.9 support and update to Python 3.14

[tboy1337]

Summary

https://devguide.python.org/versions

This PR drops official support for Python 3.9 following its end-of-life on October 5, 2025, and updates the minimum supported Python version to 3.10. Additionally, it updates the CI configuration to test against Python 3.14 (now released) instead of 3.14-dev.

Changes

Version Support Updates:

• Updated minimum Python version from 3.9 to 3.10
• Removed Python 3.9 from CI test matrix
• Updated Python 3.14-dev to 3.14 in CI workflow

Files Modified:

• .github/workflows/run-tests.yml: Updated test matrix to use Python 3.10-3.14 and removed 3.9
• README.md: Updated documentation to reflect Python 3.10+ requirement
• docs/index.rst: Updated documentation to reflect Python 3.10+ requirement
• setup.py: Updated REQUIRED_PYTHON to (3, 10), python_requires to >=3.10, and removed the Python 3.9 classifier
• tox.ini: Removed py39 from the test environment list

Rationale

According to PEP 596, Python 3.9 reached its end-of-life on October 5, 2025. This change aligns the Requests library with the official Python support lifecycle and ensures it's only supporting actively maintained Python versions.

Testing

CI tests now run on Python 3.10, 3.11, 3.12, 3.13, 3.14, PyPy 3.10, and PyPy 3.11 across Ubuntu, macOS, and Windows platforms.