[issue19172] selectors: add keys() method

[STINNER Victor]

STINNER Victor added the comment:

Using .keys() to test if the selector is empty is surprising. To test if a
str, list, tuple, dict, set, ..., is empty: if container: <not empty> is
enough.

Or sometimes I write if len(container): ...

Selector has no length?

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