Make boto3 and zarr extra dependencies

[jwodder]

These dependencies are very large and are irrelevant to the project's core features (to say nothing of the times that the boto3 requirement has sent me into dependency hell).

@AlmightyYakob I would advise releasing this change as a minor version update.

[jjnesbitt]

@jwodder Based on the discussion in dandi/dandi-cli#1371 (comment), I wonder if we should first remove the dependency on zarr (#41), and then rebase this PR to simply make boto3 an extra requirement.

[yarikoptic]

I think having a walker which we might even reuse in dandi-cli would indeed be useful and remove dependency on zarr. @jjnesbitt would you want to do it or us to do it?

[jjnesbitt]

I think having a walker which we might even reuse in dandi-cli would indeed be useful and remove dependency on zarr. @jjnesbitt would you want to do it or us to do it?

What you're talking about specifically is removing the use of the zarr package in yield_files_local, and replacing it with a native os.walk implementation.

I think that makes sense, however I think that should be addressed separately. As far as this PR, I'll merge this to take care of the issue at hand. Whenever we remove the zarr dependency, we can bump another minor version.