Test dependency fixes

[BorisTheBrave]

I had to add these dependencies I'm unclear how you found things working without these changes.

1. Since v1.8, h5netcdf does not install h5py by default. This lets you swap back ends, which is cool. But io is meant to be a quick shorthand, I doubt users want stuff to be broken by default.

2. pytz is now an optional dependency of pandas.. xarray uses it for a single test. Potentially that test should be dropped or swapped for zoneinfo, but this was most straightforward.

NB: I also had to install tzdata before the pytz test worked. But I think that is specific to my setup, I think timezone data is available commonly enough that it doesn't make sense to add a dependency.

[welcome]

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