Zarr is not optional

[ricardoV94]

Description

Zarr is not part of the requirements but pymc now imports it explicitly

[lucianopaz]

What?! This must have happened because of all the rebases that we did at the end. I'll change this ASAP.

[ricardoV94]

Also installing Zarr is not enough, it fails to do from zarr.storage import BaseStore, is this another dependency?

[ricardoV94]

Ah, the second error is because I installed 3.0,

[lucianopaz]

My bad actually. There were some conditions that I hadn't caught when working on #7540. I've now tested on a clean new environment without zarr and pymc imports well and works.