Compatibility with former h5py library

[kif]

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ERROR: test_nexus (pyFAI.test.test_io.TestSpotWriter)
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Traceback (most recent call last):
  File "/home/tester/pyFAI/build/lib.linux-x86_64-3.7/pyFAI/test/test_io.py", line 183, in test_nexus
    io.spots.save_spots_nexus(tmpfile, self.spots, beamline="beamline", ai=self.ai)
  File "/home/tester/pyFAI/build/lib.linux-x86_64-3.7/pyFAI/io/spots.py", line 124, in save_spots_nexus
    sparsify_grp.create_dataset("argv", data=numpy.array(sys.argv, h5py.string_dtype("utf8"))).attrs["help"] = "Command line arguments"
AttributeError: module 'h5py' has no attribute 'string_dtype'