[issue2928] Allow set/frozenset for __all__

[Raymond Hettinger]

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

The breakage isn't from the patch.  The problem would come from using a 
non-list iterable for __all__ which then breaks any other code that 
assumes __all__ is a list (the code is os is an example of how __all__ 
can get modified).  

AFAICT, this proposal doesn't add value, but it does complicate life 
for those wanting to access/modify __all__ but can no longer make 
assumptions about its type (perhaps it is a generator that can be read 
just once and any accessor would kill it in the process of examining 
it).  I don't see how this proposal adds value.  It's just a random 
change that doesn't make life any better for coders.

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