Add optional dependencies for preconf libraries

[alexomics]

This commit adds optional dependencies for the preconf libraries, allowing easier install for downstream users. E.G.:

pip install cattrs[tomlkit]

and (I hope) pyproject.toml:

[project]
name = "some-package"
version = "1"
dependencies = [
    "cattrs[ujson,tomlkit]"
]

It's a small change that moves the preconf dependencies out of the dev group and marks them as optional/extra. It adds an optional dependency on each library and a preconf option that installs all extras.

The change to the tox.ini is needed as I couldn't work out how, with poetry, to add the preconf dependencies back in so that they weren't duplicated.

I'm sure that I will need to update some docs for this, but I'm not sure where would be best!

[Tinche]

Very cool, I'm in favor. Although I think the preconf extra is overkill and will be an attractive nuisance, bloating up people's dep lists.

Let's add a change log entry for sure, and to the preconf docs page.

[alexomics]

Thanks for the quick reply! I've added some explanation to the preconf docs and a changelog entry. Let me know if you think it needs more detail

[Tinche]

Looks like we have test failures, let's get that fixed and merge this in! Good job!

[Tinche]

Sweet, thanks!