Test added and patch to fix python-version-dependent issues when len ro...
...w/col_levels is 1.

The docs for sparse to_coo methods failed to build. There was some case (row_levels len 1) that failed in python 2.7 only that I failed to test (and I have been building docs in python 3). Have added test and patched. Also needed to expand the interator (list(map(... ) in the "# squish" line as there was some tupleizing differences between python 3 and 2.

Perhaps there is a better way to avoid these issues? Waiting for Travis.
