BUG: fix build with single-threaded python

Some symbols are not defined leading to:
ImportError Undefined symbol: PyGILState_Release

Closes gh-3255
Merge pull request #4206 from juliantaylor/single-thread-fix

BUG: fix build with single-threaded python
BUG: Initialize NpyIter object to zero after malloc

Unitialized NpyIter object is causing random weird behavior, including object's add method being invoked when sum() is called on an array of a single object. This results in the single object in the array being added to itself which is incorrect.