v1.6.3 - Reinstate `h5py>=3.12`
Bugfix
#28
- h5py v3.12.0 was actually fine, but we did need to change the way that
the hdf5 tests work to not hold the file open during the test. Easy enough change.
the version cap has been removed from h5py (which is optional anyway,
so any version could be installed separately)