Default opening mode

[clbarnes]

In h5py v2.10 (2019-09-06), opening without an explicit mode raises a deprecation warning (which can be silenced with an environment variable or global config setting). From h5py 3.0, the default mode will be readonly. https://docs.h5py.org/en/latest/whatsnew/2.10.html#deprecations

pyn5 (/h5py_like) may want to do the same for compability / because readonly is the sensible default anyway.