feat: Include abiflags in version dectection (enabling 3.13t)

[effigies]

Description

Currently, you can install free-threaded Python in GitHub actions (with deadsnakes or uv), and have tox.ini contain:

[gh-actions]
python =
  3.9: py39
  3.10: py310
  3.11: py311
  3.12: py312
  3.13: py313
  3.13t: py313t

But the version is detected as 3.13, since only the version information is considered, not the abiflags. This enables abiflags, which in turn enables this use case.

This is important for setting conditions in tox that only apply to free-threading builds, such as setting PYTHONGIL=0 or limiting dependencies to those with py3 or cp313t wheels.

You can see it in action in https://github.com/nipy/nibabel/actions/runs/12217554525/job/34081969908:

ROOT: 226 I running tox-gh-actions [tox_gh_actions/plugin.py:28]
ROOT: 227 D original envlist: ['py39-none', 'py310-none', 'py311-none', 'py312-none', 'py313-none', 'py313t-none', 'py39-min', 'py39-full', 'py39-full-x86', 'py39-full-x64', 'py39-pre-x86', 'py39-pre-x64', 'py310-full-x86', 'py310-full-x64', 'py310-pre-x86', 'py310-pre-x64', 'py311-full-x86', 'py311-full-x64', 'py311-pre-x86', 'py311-pre-x64', 'py312-full-x64', 'py312-pre-x64', 'py313-full-x64', 'py313-pre-x64', 'py313t-full-x64', 'py313t-pre-x64', 'py313-dev-x64', 'install', 'doctest', 'style', 'typecheck'] [tox_gh_actions/plugin.py:42]
ROOT: 227 D Python versions: ['3.13t', '3.13', '3'] [tox_gh_actions/plugin.py:45]
ROOT: 228 D tox-gh-actions config: {'python': {'3.9': ['py39'], '3.10': ['py310'], '3.11': ['py311'], '3.12': ['py312'], '3.13': ['py313'], '3.13t': ['py313t']}, 'env': {'ARCH': {'x64': ['x64'], 'x86': ['x86'], 'arm64': ['arm64']}, 'DEPENDS': {'none': ['none', 'install'], 'pre': ['pre'], 'dev': ['dev'], 'full': ['full', 'install'], 'min': ['min']}}} [tox_gh_actions/plugin.py:48]
ROOT: 228 D got factors for Python version: 3.13t [tox_gh_actions/plugin.py:137]
ROOT: 228 D using the following factors to decide envlist: ['py313t-x64-full', 'py313t-x64-install'] [tox_gh_actions/plugin.py:51]
ROOT: 228 D expiring envlist cache to override [tox_gh_actions/plugin.py:121]

Expected Behavior

When detecting a matching Python, 3.13t will be tried before falling back to 3.13 and 3.

[effigies]

This will break Windows, which doesn't have this attribute, unfortunately. It looks like it can be recovered from EXT_SUFFIX, which would be a pretty ugly hack, but I can do it...