Don't run pytest-run-parallel twice

[crusaderky]

Follow-up to #507.
Do not run the 'parallel' tox environment (which is hardcoded to 3.14t) twice unnecessarily:

• once on the 3.13t CI job (but it runs python 3.14t)
• once on the 3.14t CI job

[davidism]

It seems like adding 3.13t to base_python for this env may do the right thing. I want to avoid hard coding specific versions in more places, such as CI, especially outside the matrix, because it's more likely to be missed when changing what versions we test.

[crusaderky]

apologies, my tox/uv knowledge is very limited (I normally use pixi) and can't figure this out.

If I change pyproject.toml to

[tool.tox.env.parallel]
description = "check for free threading issues"
base_python = ["3.13t", "3.14t"]
commands = [[
    "pytest", "-v", "--tb=short", "--basetemp=env_tmp_dir",
    "--parallel-threads=8",
    {replace = "posargs", default = [], extend = true},
]]

then uv run --locked tox run -e parallel always runs on 3.13t.
Is there a command syntax (which I can't find in the documentation) to force 3.14t? Or should I just duplicate the environment?

[tool.tox.env.parallel-3.13t]
description = "check for free threading issues"
base_python = ["3.13t"]
commands = [[
    "pytest", "-v", "--tb=short", "--basetemp=env_tmp_dir",
    "--parallel-threads=8",
    {replace = "posargs", default = [], extend = true},
]]
[tool.tox.env.parallel-3.14t]
description = "check for free threading issues"
base_python = ["3.14t"]
commands = [[
    "pytest", "-v", "--tb=short", "--basetemp=env_tmp_dir",
    "--parallel-threads=8",
    {replace = "posargs", default = [], extend = true},
]]