make nditer also a context manager

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make nditer also a context manager

mattip
When an nditer uses certain op_flags[0], like updateifcopy or readwrite
or copy, the operands (which are ndarray views into the original data)
must use the UPDATEIFCOPY flag. The meaning of this flag is to allocate
temporary memory to hold the modified data and make the original data
readonly. When the caller is finished with the nditer, the temporary
memory is written back to to the original data and the original data's
readwrite status is restored. The trigger to resolve the temporary data
is currently via the deallocate function of the nditer, thus the call
becomes something like

i = nditer(a, op_flags=<flags>)
#do something with i
i = None # trigger writing data from i to a

This IMO violates the "explicit is better" philosophy, and has the
additional disadvantage of relying on refcounting semantics to trigger
the data resolution, which does not work on PyPy.

I have a pending pull request[1] to add a private numpy API function
PyArray_ResolveUpdateIfCopy, which allows triggering the data resolution
in a more explicit way. The new API function is added at the end of the
functions like take or put with non-contiguous arguments, explicit tests
have also been added . The only user-facing effect is to allow using an
nditer as a context manager, so the lines above would become

with nditer(a, op_flags=<flags>) as i:
     #do something with i
# data is written back when exiting

The pull request passes all tests on CPython. The last commit makes the
use of a nditer context manager mandatory on PyPy if the UPDATEIFCOPY
semantics are triggered, while allowing existing code to function
without a warning on CPython.

Note that np.nested_iters[2] is problematic, in that it currently
returns a tuple of nidters, which AFAICT cannot be made into a context
manager, so __enter__ and __exit__ must be called manually for the nditers.

At some future point we could decide to deprecate the non-context
managed use on CPython as well, following a cycle of first issuing a
deprecation warning for a few release versions.

Any thoughts? Does making nditer a context manager make sense? How
widespread is use of nested_iters in live code (it does not appear in
the official NumPy documentation)
Thanks,
Matti

[0] https://docs.scipy.org/doc/numpy/reference/generated/numpy.nditer.html
[1] https://github.com/numpy/numpy/pull/9269
[2]
https://github.com/numpy/numpy/blob/master/numpy/core/tests/test_nditer.py#L2344 

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