python3Packages.arviz: patch requirements.txt and add zarr tests
python3Packages.arviz: patch requirements.txt and add zarr tests

arviz v0.11.4 has, according to its requirements.txt, a dependency on
typing-extensions < 4. Currently, only typing-extensions v.4.0.1 are
available in nixpkgs, while the above version constraint seems
unnecessary, see this arviz PR: https://github.com/arviz-devs/arviz/pull/1948/files.
This also activates tests that use zarr and to which changes are made
in above PR. I thought those changes might be related to
unconstraining the typing-extensions version, but looks like they are not.

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