h5py removed .value after v3

[indebetouw]

in v3, value was removed from Structure.
these changes instead return strings for newick and wcs_header, and numpy arrays for data and index_map, when reading a saved hdf5 dendrogram.

[keflavich]

lgtm, but I don't know enough about hdf5 to review this usefully. Anyone else want to chime in?

(we really need tests / CI)

[indebetouw]

I don't know what level of compatibility you want to maintain with older versions of h5py, or even with Python2. I did test with both h5py v>3 and v<3, but I did not test with Python2. In particular the conversion of byte to string was unnecessary back in python2.

[keflavich]

We don't need to maintain any support of python 2. I'm not sure about h5py though.

[e-koch]

Might impact loading dendrograms with glue: https://github.com/glue-viz/glue/blob/ed71979f8e0e41f993a2363b3b5a8f8c3167a130/glue/plugins/dendro_viewer/data_factory.py#L81

@astrofrog?

[astrofrog]

I'll investigate!