ENH: add __contains__() to np.lib.npyio.NpzFile

[f380cedric]

NpzFile inherits from collections.abc.Mapping,
which provides __contains__().
However, it calls __getitem__(),
which can be slow because it performs file decompression on success.

[charris]

Thanks @f380cedric .

[charris]

Oops, I was a bit hasty. @f380cedric can you make PR adding a release note for this? See doc/release/upcoming_changes for other release notes to see how it is done.

[f380cedric]

Yes, sure! I misunderstood the contribution doc, sorry for that.

[InessaPawson]

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