Mark test_accessor_sliced_datacube as xfail due to upstream bug

[seisman]

Description of proposed changes

GMT 6.3.0 doesn't work on 3D grid cubes (reported in GenericMappingTools/gmt#6588 (comment)).

In GMT 6.3.0, the grdinfo output is empty:

$ gmt grdinfo eraint_uvz.nc -Cn
grdinfo [WARNING]: Detected a data cube (eraint_uvz.nc) but -Q not set - skipping

After the upstream fix GenericMappingTools/gmt#6615, the output is:

$ gmt grdinfo eraint_uvz.nc -Cn
-180	179.25	-90	90	66825.5	66825.5	0.75	0.75	480	241	0	1

From the output, it's clear that the grid cube has registration=0 and gtype=1.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

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