TOXENV=py313t doesn't work

[ngoldbaum]

Issue

Now that virtualenv has support for free-threaded Python, tox should also add support for environment names like py313t. The virtualenv release came out in the last 24 hours and that led to CI breakage on bcrypt, which was testing free-threaded python using tox.

Environment

See e.g. this CI run: https://github.com/pyca/bcrypt/actions/runs/12814705768/job/35731757807?pr=953

In that run TOXENV is set to py313t. I had it set to py313 before the last virtualenv release, which worked, to work around the lack of support in virtualenv.

Output of running tox

Output of tox -rvv

ROOT: 209 E HandledError| provided environments not found in configuration file:
cp313t [tox/run.py:23]

Minimal example

Clone the bcrypt repo and run tox with a free-threaded python installed and TOXENV set to py313t. You'll need a rust toolchain installed.

[gaborbernat]

Duplicate of #3391