Add support for free-threading Python builds

[ymyzk]

Description

This change adds support for Python's free-threading builds (e.g., Python 3.14t) by detecting the Py_GIL_DISABLED configuration variable and appending a "t" suffix to version keys when running on a free-threading build.

Changes include:

• Updated plugin to check sysconfig.get_config_var("Py_GIL_DISABLED")
• Added "t" suffix to version keys for free-threading builds
• Updated documentation with example configuration
• Added test case for free-threading Python version detection

Expected Behavior

The following example should run py314 on a normal Python 3.14 job and run py314t on a free-threaded Python 3.14 job.

[tox]
envlist = py314, py314t

[gh-actions]
python =
    3.14: py314
    3.14t: py314t

By design, the t suffix is needed even when only free-threaded Python is installed on a job. For example, the following configuration fails to run python3.14t if python3.14t is installed but python3.14 is not installed.

[tox]
envlist = py314

[gh-actions]
python =
    3.14: py314

[Copilot]

Pull Request Overview

This PR adds support for Python's free-threading builds (introduced in Python 3.13+) by detecting the Py_GIL_DISABLED configuration variable and appending a "t" suffix to version keys when running on a free-threading build.

• Detects free-threading builds using sysconfig.get_config_var("Py_GIL_DISABLED")
• Appends "t" suffix to all Python version keys (CPython, PyPy, Pyston) when free-threading is enabled
• Updates documentation with example configuration for py314t environment

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/tox_gh_actions/plugin.py	Adds free-threading detection and appends "t" suffix to version keys when Py_GIL_DISABLED == 1
tests/test_plugin.py	Adds test case for free-threading Python 3.14 version detection with mocked Py_GIL_DISABLED config variable
README.md	Documents free-threading Python configuration with example showing py314 and py314t environments

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