BUG: fix error message when trying to load a Parthenon dataset without h5py installed

[neutrinoceros]

PR Summary

Fix a small oversight from #4323

reprod:
install pooch, pandas, uninstall h5py

import yt
yt.load_sample("parthenon_advection")

error message on main:

Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    yt.load_sample("parthenon_advection")
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/clm/dev/yt-project/yt/yt/loaders.py", line 1684, in load_sample
    return load(loadable_path, **load_kwargs)
  File "/Users/clm/dev/yt-project/yt/yt/_maintenance/deprecation.py", line 68, in inner
    return func(*args, **kwargs)
  File "/Users/clm/dev/yt-project/yt/yt/loaders.py", line 141, in load
    return cls(fn, *args, **kwargs)
  File "/Users/clm/dev/yt-project/yt/yt/frontends/parthenon/data_structures.py", line 153, in __init__
    self._handle = HDF5FileHandler(filename)
                   ~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/Users/clm/dev/yt-project/yt/yt/utilities/file_handler.py", line 32, in __init__
    self.handle = h5py.File(filename, mode="r")
                  ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "/Users/clm/dev/yt-project/yt/yt/utilities/on_demand_imports.py", line 39, in __call__
    raise self.error
  File "/Users/clm/dev/yt-project/yt/yt/utilities/on_demand_imports.py", line 77, in inner
    return func(self)
  File "/Users/clm/dev/yt-project/yt/yt/utilities/on_demand_imports.py", line 305, in File
    from h5py import File
ModuleNotFoundError: No module named 'h5py'
Something went wrong while trying to lazy-import h5py. Please make sure that h5py is properly installed.
If the problem persists, please file an issue at https://github.com/yt-project/yt/issues/new

this branch

Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import yt; yt.load_sample('parthenon_advection')
               ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/clm/dev/yt-project/yt/yt/loaders.py", line 1684, in load_sample
    return load(loadable_path, **load_kwargs)
  File "/Users/clm/dev/yt-project/yt/yt/_maintenance/deprecation.py", line 68, in inner
    return func(*args, **kwargs)
  File "/Users/clm/dev/yt-project/yt/yt/loaders.py", line 146, in load
    raise YTUnidentifiedDataType(_input_fn, *args, **kwargs)
yt.utilities.exceptions.YTUnidentifiedDataType: Could not determine input format from PosixPath('/Users/clm/dev/yt-project/test-data-dir/parthenon_advection/advection_2d.out0.final.phdf')
The following types could not be thorougly checked against your data because their requirements are missing. You may want to inspect this list and check your installation:
- ArepoHDF5Dataset (requires: h5py)
- CFRadialDataset (requires: xarray, pyart)
- CM1Dataset (requires: netCDF4)
- ChimeraDataset (requires: h5py)
- ChollaDataset (requires: h5py)
- ChomboDataset (requires: h5py)
- ChomboPICDataset (requires: h5py)
- EagleDataset (requires: h5py)
- EagleNetworkDataset (requires: h5py)
- EnzoDataset (requires: h5py)
- EnzoDatasetInMemory (requires: h5py)
- EnzoEDataset (requires: h5py, libconf)
- EventsFITSDataset (requires: astropy)
- ExodusIIDataset (requires: netCDF4)
- FITSDataset (requires: astropy)
- FLASHDataset (requires: h5py)
- FLASHParticleDataset (requires: h5py)
- GAMERDataset (requires: h5py)
- GDFDataset (requires: h5py)
- GadgetFOFDataset (requires: h5py)
- GadgetHDF5Dataset (requires: h5py)
- GizmoDataset (requires: h5py)
- MoabHex8Dataset (requires: h5py)
- OWLSDataset (requires: h5py)
- OWLSSubfindDataset (requires: h5py)
- OpenPMDDataset (requires: h5py)
- OpenPMDGroupBasedDataset (requires: h5py)
- Orion2Dataset (requires: h5py)
- ParthenonDataset (requires: h5py)
- PlutoDataset (requires: h5py)
- SkyDataFITSDataset (requires: astropy)
- SpectralCubeFITSDataset (requires: astropy)
- SwiftDataset (requires: h5py)
- YTClumpTreeDataset (requires: h5py)
- YTDataContainerDataset (requires: h5py)
- YTDataLightRayDataset (requires: h5py)
- YTFITSDataset (requires: astropy)
- YTGridDataset (requires: h5py)
- YTHaloCatalogDataset (requires: h5py)
- YTNonspatialDataset (requires: h5py)
- YTProfileDataset (requires: h5py)
- YTSpatialPlotDataset (requires: h5py)

Please make sure you are running a sufficiently recent version of yt.

PR Checklist

• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

[pgrete]

Looks good to me (in case I can approve)