`iinfo()` and `finfo()` result holds `dtype` attribute

[honno]

PR which depends on #484

In yesterdays meeting @oleksandr-pavlyk bought up that NumPy's iinfo()/finfo() returns info objects with a dtype attribute. ~There seemed to be no objections, but it turns out dtype wasn't actually document for NumPy...~ Resolved in https://github.com/numpy/numpy/pull/22334

I've left this PR separate from #484 to keep things atomic, incase things become finicky. It depends on that PR because of the subtle distinction I made in finfo() to clarify attributes relate to real-valued floats... just want that to resolve (i.e. be merged) first.