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.

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[jsignell]

Thanks for putting this PR together! It seems reasonable to me. I think the reason why this wasn't caught in CI is because the dependencies for each testing environment is unrelated to these optional dependency lists. If you look at pixi.toml you'll see that h5py is explicitly included in the dependencies for backend tests.

Now I am not quite sure why this is failing tests. Might need @VeckoTheGecko to advise on that.

[VeckoTheGecko]

Ah ! CI failure due to Parcels-code/pixi-lock#14

[VeckoTheGecko]

CI is running again :)