[issue19172] selectors: add keys() method

[Berker Peksag]

Berker Peksag added the comment:

+   .. method:: get_map()
+
+      Return a mapping of file objects to selector keys.

I kind of feel like a walking linter though I think this needs a versionadded tag :)

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keywords:  -needs review
nosy: +berker.peksag

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19172>
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