env_list and env_base.factors: enable labels for 'Bare range dict'

[nathanwilliams-ct]

It might be nice if we could add a label to a 'Bare range dict' e.g.:

factors = [
  { prefix = "3.", start = 12, stop = 14 }, 
  ["django42", "django50"]
]

description = "{factor.0} some text {factor.1}"

might become:

factors = [
  { py_version = { prefix = "3.", start = 12, stop = 14 }}, # not possible: `TypeError: labeled factor group 'py_version' must map to a list, got dict`
  { django_version = ["django42", "django50"]} # currently possible
]

description = "{factor.py_version} some text {factor.django_version}"

or

factors = [
  { prefix = "3.", start = 12, stop = 14, label = "py_version" }, # not possible
  { django_version = ["django42", "django50"]} # currently possible
]

description = "{factor.py_version} some text {factor.django_version}"

https://tox.wiki/en/4.60.1/reference/config.html#toml-env-list-item-shapes
https://tox.wiki/en/4.60.1/reference/config.html#factor-label-substitution

[gaborbernat]

#4049 implements this, and it goes out in 4.61. A range takes a label by nesting it under one, the same way a list does:

[env_base.test]
factors = [
  { py_version = { prefix = "3.", start = 12, stop = 14 } },
  { django_version = ["django42", "django50"] },
]
description = "{factor:py_version} some text {factor:django_version}"

This generates test-3.12-django42 through test-3.14-django50, and {factor:py_version} resolves to whichever version the environment carries.

The { prefix = "3.", start = 12, stop = 14, label = "py_version" } form from the second half of your report did not ship. A label key suits a range but has nowhere to live in a list, so it would have named one shape and left the other out. Nesting covers both with one rule. Reopen if you want the key form too.