Remove viewitems() as it breaks compatibility with python<2.7

[foobacca]

It was introduced in commit 808768c -
adding support for Solr's JSON API.

viewitems() was backported from Python 3 items() into python 2.7, but if
we use it then python 2.6 and older will fail.

[evansd]

Hi @foobacca, these are only used in the JSON methods so the original XML API should still be Python 2.6 compatible.

We (I work with @tow, the original author) added support for the JSON API because we're working with very large results sets and need to be as efficient (in terms of CPU and memory usage) as possible. For this reason we'd like to stick with using viewitems() to avoid the overhead of creating new lists.

The JSON API doesn't offer any additional features over the XML one, just better performance with large result sets, so if you're stuck on Python 2.6 I'd suggest just sticking with the XML API.

[foobacca]

Would iteritems() (python 2.2+) be an acceptable compromise? That is still a generator. The main difference between viewitems() and iteritems() that I can see, is that when you use viewitems() the generator will reflect changes to the underlying dict after the for loop has started. But I don't see any advantage to that in the places you're using them.

If you're happy with iteritems() then let me know and I'll do another pull request.