BUG: Ordering of dtypes is ill defined

[seberg]

The ordering of DTypes is ill defined, due to the fact that equality is much strong than established by casting (forcing the same byte order):

In [1]: f8b = np.dtype(">f8")  
In [2]: f8l = np.dtype("<f8")

In [3]: f8l <= f8b
Out[3]: True

In [4]: f8l <= f8b
Out[4]: True

In [5]: f8l == f8b
Out[5]: False

which is not transitive. I am not sure this is a huge issue, but maybe there is just no point in defining < and > based on casting for dtypes, since can_cast is reasonable and it is not a common operation.
The meaning of == should be set in stone, just due to the amount of code that will be subtly wrong if it does not enforce native byte order anymore.

[seberg]

I am not super concerned about this, but I personally doubt the use and usefulness of operator comparisons, so I feel we might as well deprecate ordering operators (you still have can_cast to establish ordering if you want to).