Bug? numpy.dtype is not sortable.
I find this when I am writing the pull request #1098.  As it is loosely related to the pull request, I am reporting here as a separate issue.

It looks like dtype does not support sorting:

``` python
>>> from numpy import dtype
>>> sorted([dtype('bool'), dtype('float64'), dtype('complex64'),
...         dtype('float64'), dtype('object')])
[dtype('bool'),
 dtype('float64'),
 dtype('complex64'),
 dtype('float64'),
 dtype('object')]
```

`core.internals._consolidate` relies sortability of `dtype`.  Probably it is better to sort using `dtype.name` as I did in bae81555be2c16cb191b5c9f2b9f9f305eea53d5?
