spurious error with __slots__ and unicode_literals

[pylint-bot]

Originally reported by: BitBucket: andreasvc, GitHub: @andreasvc?

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When unicode_literals is enabled, an error is reported that only strings are acceptable in __slots__:

Invalid object "u'foo'" in __slots__, must contain only non empty strings (invalid-slots-object)

However, unicode strings in slots are acceptable in Python 2, and using unicode helps in writing code that is compatible in both Python 2 and 3.

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• Bitbucket: https://bitbucket.org/logilab/pylint/issue/421

[pylint-bot]

Original comment by Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore):

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Fix a false positive with invalid-slots-objects and unicode strings. Closes issue #421.