chore(deps): make fsspec and upath optional dependencies

[jhamman]

This PR makes a few minor changes to our project dependencies and test imports.

closes #2391
Fixes #735
Supersedes #2395

[dstansby]

This is a big change in user facing behaviour when the package is installed, so should definitely have a prominent entry in the what's new/changed for 3.0 docs!

This probably also needs a change to the installation docs to explain how to install packages for remote data access.

[jhamman]

This is a big change in user facing behaviour when the package is installed, so should definitely have a prominent entry in the what's new/changed for 3.0 docs!

This is not quite right. Fsspec is an optional dependency for Zarr-Python 2 as well. I'm planning to do a rev on the docs next week so will include this then.

[dstansby]

This is not quite right. Fsspec is an optional dependency for Zarr-Python 2 as well. I'm planning to do a rev on the docs next week so will include this then.

Ah my bad. I think it's still worth adding a quick changelog note somewhere that this is a change in behaviour from the current beta release - shouldn't take much time to add, and I think adds a lot of value for users if we have a user-facing log of changes like these, even if from beta to beta to release.

[dstansby]

👍 thanks!