[FIX] unsafe listification of weakref in Python < 2.7.4

Fixes #966

* As a preallocation optimization, ``list()`` calls ``__len__`` on its
  parameter if it's available
* Before Python 2.7.4, WeakSet has a bug[0] where ``len()`` is unsafe: it is
  done by iteration and weakrefs may be removed from the underlying set during
  the iteration

As a result, the safety feature of listifying a WeakSet to ensure we have
strong refs on all items during iteration may blow up.

Wrapping the weakset in a ``iter()`` makes ``__len__()`` invisible and ensures
we're within the IterationGuard[1].

Which now that I think about it means we *should* be able to safely iterate
weaksets in the first place and may not have needed to listify them...

[0] http://bugs.python.org/issue14159
[1] http://hg.python.org/cpython/file/b6acfbe2bdbe/Lib/_weakrefset.py#l58