[issue18962] Add special case for single iterator in heapq.merge function

[Roundup Robot]

Roundup Robot added the comment:

New changeset 0e70bf1f32a3 by Raymond Hettinger in branch 'default':
Issue #18962:  Optimize the single iterator case for  heapq.merge()
http://hg.python.org/cpython/rev/0e70bf1f32a3

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nosy: +python-dev

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