BUG: Series.iteritems should be lazy

[TomAugspurger]

Closes #6760

[jreback]

this seems fine
ok when green

[immerrr]

A minor nitpick: the test won't catch lists in py3, because getslice is deprecated since py2.6 and is removed in py3:

In [3]: sys.version
Out[3]: '3.3.3 |Continuum Analytics, Inc.| (default, Nov 25 2013, 17:28:04) \n[GCC 4.1.2 20080704 (Red Hat 4.1.2-54)]'

In [4]: list()
Out[4]: []

In [5]: _4.__getslice__
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-ff32cb569e0e> in <module>()
----> 1 _4.__getslice__

AttributeError: 'list' object has no attribute '__getslice__'

This is probably not an issue within this PR, since it was fixing a py2-specific inconsistency, but worth keeping an eye on.

[TomAugspurger]

Thanks. I didn't realize it was deprecated. I'll switch to checking for reverse.