Import h5netcdf.core to use EnumType (#10779)

* Import EnumType from h5netcdf.core directly

* Support older h5netcdf

---------

Co-authored-by: Kai Mühlbauer <[email protected]>
Co-authored-by: Kai Mühlbauer <[email protected]>

Author: maxrjones

xarray/backends/h5netcdf_.py

@@ -260,7 +260,7 @@ def ds(self):
         return self._acquire()
 
     def open_store_variable(self, name, var):
-        import h5netcdf
+        import h5netcdf.core
         import h5py
 
         dimensions = var.dimensions