Hello !

First, let me thank you for providing such powerful tool. It has greatly helped simplifying CI in pyvista

It seems that this PR now prevents using complex factor conditions as described in https://tox.wiki/en/3.27.1/config.html#complex-factor-conditions
I recognize it was documented in an old version of tox that disappeared in latest docs (#3276) but it was working as I expected it before this PR.

Here is MWE with the behavior changes induced by this PR:

[tox]

env_list = py3.{10-14}

[testenv]

deps =
    matplotlib
    np: numpy
    np-cov: coverage

We managed to find a workaround in pyvista/pyvista#8343 by explicitly declaring factors but I was curious if this behavior change introduced by this PR was intentional.

Thanks in advance.

Originally posted by @edabor in #3753 (comment)