Xarray: fix compatibility with h5py version 3 and unpin tests

[mathause]

h5py version 3.1 broke our tests. I pinned it to version 2.10 in #4569. We should therefore

• fix the issues
• unpin h5py again

The failures could be related to a change how strings are read: https://docs.h5py.org/en/latest/strings.html I am not sure if this has to be fixed in xarray or in h5necdf. I'd be happy if someone else took this one.

Failed tests:

FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_zero_dimensional_variable
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_write_store - As...
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_roundtrip_test_data
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_load - Assertion...
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_dataset_compute
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_roundtrip_object_dtype
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_roundtrip_string_data
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_orthogonal_indexing
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_vectorized_indexing
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_isel_dataarray
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_array_type_after_indexing
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_append_write - A...
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_append_overwrite_values
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_write_groups - A...
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_encoding_kwarg_vlen_string
FAILED xarray/tests/test_backends.py::TestH5NetCDFData::test_compression_encoding
FAILED xarray/tests/test_backends.py::TestH5NetCDFFileObject::test_zero_dimensional_variable
FAILED xarray/tests/test_backends.py::TestH5NetCDFFileObject::test_write_store
FAILED xarray/tests/test_backends.py::TestH5NetCDFFileObject::test_roundtrip_test_data
FAILED xarray/tests/test_backends.py::TestH5NetCDFFileObject::test_load - Ass...
FAILED xarray/tests/test_backends.py::TestH5NetCDFFileObject::test_dataset_compute
FAILED xarray/tests/test_backends.py::TestH5NetCDFFileObject::test_roundtrip_object_dtype
FAILED xarray/tests/test_backends.py::TestH5NetCDFViaDaskData::test_encoding_kwarg_vlen_string
FAILED xarray/tests/test_backends.py::TestH5NetCDFViaDaskData::test_compression_encoding
FAILED xarray/tests/test_distributed.py::test_dask_distributed_netcdf_roundtrip[h5netcdf-NETCDF4]
FAILED xarray/tests/test_distributed.py::test_dask_distributed_read_netcdf_integration_test[h5netcdf-NETCDF4]

Example failure:

```python traceback

assert_allclose(original, computed)
E AssertionError: Left and right Dataset objects are not close
E
E Differing coordinates:
E L * dim3 (dim3) E R * dim3 (dim3) object b'a' b'b' b'c' b'd' b'e' b'f' b'g' b'h' b'i' b'j'<
```

[kmuehlbauer]

@mathause A quick check running h5netcdf testsuit suggest it needs fixing in h5netcdf. I'll have a closer look.

[kmuehlbauer]

The longer I look at that trying to find the differences the more confused I get. Any hints appreciated. Trying to catch up tomorrow.

[kmuehlbauer]

I opened an issue over at h5netcdf (https://github.com/shoyer/h5netcdf/issues/81) with my current evaluations.

[kmuehlbauer]

This should possibly be resolved by https://github.com/shoyer/h5netcdf/pull/82. At least the xarray testsuite runs with that h5netcdf-patch for all versions of h5py.