Before:
```
In [1]: np.dtype({"names": ["a"], "formats": [int], "offsets": [2]})
Out[1]: dtype({'names':['a'], 'formats':['<i8'], 'offsets':[2], 'itemsize':10})
```

After:
```
In [1]: np.dtype({"names": ["a"], "formats": [int], "offsets": [2]})
Out[1]: dtype({'names': ['a'], 'formats': ['<i8'], 'offsets': [2], 'itemsize': 10})
```

* Allow switching back to old dtype printing format.

* Add changelog.

* DOC: Fix typos in versionchanged.

This PR uses simple cases of PEP 380 to rewrite:

for v in g:
    yield v
into:

yield from <expr>

This replaces basestring with str except in

- tools/npy_tempita/
- numpy/compat/py3k.py

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

BUG: Fix flatten_dtype so that nested 0-field structs are flattened correctly

This affects the behavior of merge_arrays.