[issue2928] Allow set/frozenset for __all__

[Raymond Hettinger]

Raymond Hettinger <rhettinger at users.sourceforge.net> added the comment:

Does this break code that adds to __all__ using append() and extend()?  
See the os module for an example.

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assignee:  -> rhettinger
nosy: +rhettinger
type:  -> feature request
versions: +Python 2.6

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