Clean up optional dependencies

[dstansby]

On the current main branch, there are "extra" and "optional" optional dependecy groups. These names aren't very descriptive, so over in #2538 I wasn't sure how to documente them. It would be good if we moved or renamed these dependencies. Does anyone have any suggestions?

[project.optional-dependencies]
...
extra = [
    'msgpack',
]
optional = [
    'universal-pathlib>=0.0.22',
    'rich'
]
...

[d-v-b]

Do msgpack, rich or universal-pathlib interact in the codebase? I am guessing that they are pretty independent, which means that they probably shouldn't be grouped. Maybe one dependency group for each one is the way to address this?

[dstansby]

It looks like:

• msgpack is only used in the docs (?), so we can remove it from "extra" (and remove "extra")
• universal-pathlib is used once, so we can move to a "universal-pathlib" extra
• rich is only used for tree, so we can get rid of it from "optional"

How does that sound?