Allow pathlib.Path to be passed to all engines

[alexamici]

• Related to Flexible Backend - AbstractDataStore definition  #4309 and in particular: Flexible Backend - AbstractDataStore definition  #4309 (comment)
• Passes isort . && black . && mypy . && flake8
• No user visible change

Once ZarrStore.open_group and NetCDF4DataStore.open accepts pathlib.Path objects we can remove _normalize_path from apiv2.open_dataset. All other backends accept them.

Note that zarr itself doesn't support them, yet. See: zarr-developers/zarr-python#601

[max-sixty]

LGTM @alexamici !

Could we add a small test / param onto an existing test? Or do you want to handle that as part of the larger PRs?

[alexamici]

@max-sixty there's no direct test for the backend Store classes, only indirect tests through open_dataset. For example all other beckends do support pathlib.Path but none is tested.

Once we remove _normalize_path (PR is coming, and we may need a bit of discussion before approval) we will be able to test all backends.

I propose to merge this PR as it is.

[max-sixty]

Great! Thanks!

[clbarnes]

Zarr's convenience methods accept Path objects as of 2.9.0: https://zarr.readthedocs.io/en/stable/release.html#enhancements