HDF5 string attributed are not encoded

From hyp5, tags like NXroot/creator are not str but bytes:

In [4]: h5py.File("output_sparse_0_02470.h5").attrs["creator"]
Out[4]: b'LIMA2-1.0.0'

In [5]: h5py.File("../../ID11/Vanadinite/vanadinite/vanadinite_sx_0p25s_0p3s_16Be/scan0001/eiger_0000.h5").attrs["creator"]
Out[5]: 'LIMA-1.9.10'

This is a "regression" from Lima1 reported by @kieffer.

Converting HDF5 string attributes from fixed size to H5T_VARIABLE fixes the issue.