Bump verison of autoapi used to build docs

[dstansby]

• Bumps the version of autoapi we use to build the docs
• Pins specific versions of sphinx and autoapi, which can then be updated by dependabot. I think it's best to pin the doc build to specific versions?
• Simplifies the logic of excluding private members from the docs. This is a workaround to __all__ not respected when going from 3.0.0 to 3.1.0 readthedocs/sphinx-autoapi#481, but I think worth having regardless as a simplification of conf.py.

On the last point, this does mean that zarr.v2 and zarr.core are now in the API docs, but I figure we can think about making them private by appending a _ to the beginning in follow up PRs where we can decided where we want the various bits of API to actually live.

[jhamman]

The plan is to remove the v2 namespace here soon so I wouldn't worry too much about that one. I'm also fine renaming core to _core if that helps here.

[dstansby]

👍 - I think maybe lets get this in, then rename core > _core in another PR (it will be a big diff with all the changed imports!). Or perhaps once this is merged, we can review the public API and as part of that decide what to do with core.

[jhamman]

@dstansby - is this ready for a merge?

[dstansby]

Yep