Import h5netcdf.core to use EnumType

[maxrjones]

This PR fixes an import issue with h5netcdf.core.EnumType. Recent Python versions require explicit submodule imports, so import h5netcdf followed by h5netcdf.core.EnumType fails because the core submodule isn't automatically loaded. This was causing pickling issues with NetCDF4 files (xref developmentseed/titiler-multidim#97).

(see also, zarr-developers/zarr-python#3473 for a bug caused by the same behavior)

[maxrjones]

@kmuehlbauer would you be willing to provide some advice here? The min-deps CI check is failing because EnumType was only added in h5netcdf/h5netcdf#226 (released in v1.6.4), whereas https://github.com/pydata/xarray/blob/main/ci/requirements/min-all-deps.yml lists h5netcdf=1.3 as the oldest supported version. But, I think versions older than 1.6.4 wouldn't work with the h5netcdf backend since #9509 given the use of this new feature. I'm not sure if you'd rather add a workaround for older versions or bump the minimum supported version.

[kmuehlbauer]

@maxrjones EnumType was released with h5netcdf 1.4.0 in October 2024.

[kmuehlbauer]

This is also mentioned in a comment in the #9509 PR.

Maybe you need to conditionally import depending on version? I can have a closer look tomorrow.

[maxrjones]

@maxrjones EnumType was released with h5netcdf 1.4.0 in October 2024.

ah, ok thanks for this clarification. It seems we're close, but not quite to the 12 month mark for minimum dependencies

[kmuehlbauer]

Thanks @maxrjones , I've made a mental note, to not forget to remove this in October.