expand section names

[bruchar1]

This is a proposition to allow generative section names.

In conjecture with factors, it will allow to write more succinct config files, when using factors in the default [testenv] is not sufficient.

Let me know if you like it.

[gaborbernat]

I'm not against it, but there's a lot of parts this touches; we need to ensure it does not break anything; e.g. tox -al etc

[bruchar1]

For tox -l, it should not change anything, as this is determined by envlist.
For tox -a, it seems to work as expected, since sections are generated just after reading the ini file, before any parsing.
It shouldn't break non-generative sections, as they are left unchanged.
And all tests pass.

[gaborbernat]

Let's add documentation and changelog 👍

[bruchar1]

I added documentation. Feel free to suggest improvements. I'm just not sure what to put for the version number.

[gaborbernat]

Hello, this now has been released via https://pypi.org/project/tox/3.15.0/