Add support for python 3.14/3.14t and increase requires-python to >= 3.8

[lukeshingles]

Python 3.14 release candidates are ABI stable with the upcoming final release in October, so wheels can now be published on PyPI.

[Copilot]

Pull Request Overview

This PR updates the project to support Python 3.14 and the free-threaded Python 3.14t variant while dropping support for older Python versions by increasing the minimum requirement to Python 3.8. This modernization aligns the project with current Python ecosystem standards and prepares it for the upcoming free-threading capabilities.

• Minimum Python version increased from 3.6 to 3.8
• CI testing matrix updated to include Python 3.14 and 3.14t, removing 3.6 and 3.7
• Cython compilation configured for free-threading compatibility

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
setup.py	Updates minimum Python requirement from 3.6 to 3.8
extinction.pyx	Adds free-threading compatibility flag to Cython directives
.github/workflows/upload_to_pypi.yml	Updates cibuildwheel version and removes redundant Python setup configurations
.github/workflows/python-package-tox.yml	Updates test matrix to Python 3.8-3.14t and modernizes Ubuntu runner version

Comments suppressed due to low confidence (1)

.github/workflows/python-package-tox.yml:15

• Python 3.14 and 3.14t versions do not exist yet. Python 3.14 is not scheduled for release until October 2025, and the free-threaded variant (3.14t) is still experimental. Consider using only stable Python versions in the CI matrix.

        python: ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.14", "3.14t"]