BUG: PyArray_SelectkindConverter should be in conversion_utils.h.

Also include numpy/ndarraytypes.h, although this seems not needed in
practice.
Merge pull request #3711 from charris/backport-1.8-3709

Backport 1.8 3709
BUG: Fix 0-sized ndindex

The zerosize_ok flag to nditer was missing, so that it did not
allow for 0-sized iteration.

Closes gh-3714
Merge pull request #3717 from charris/backport-1.8-3715

Backport 1.8 3715