Add iterobject.pxd (GH-3452)

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/iterator.html).

Cython/Includes/cpython/iterobject.pxd

+cdef extern from "Python.h":
+
+    ###########################################################################
+    # Iterator Objects
+    ###########################################################################
+
+    bint PySeqIter_Check(object op)
+    # Return true if the type of op is PySeqIter_Type.
+
+    object PySeqIter_New(object seq)
+    # Return value: New reference.
+    # Return an iterator that works with a general sequence object, seq. The
+    # iteration ends when the sequence raises IndexError for the subscripting
+    # operation.
+
+    bint PyCallIter_Check(object op)
+    # Return true if the type of op is PyCallIter_Type.
+
+    object PyCallIter_New(object callable, object sentinel)
+    # Return value: New reference.
+    # Return a new iterator. The first parameter, callable, can be any Python
+    # callable object that can be called with no parameters; each call to it
+    # should return the next item in the iteration. When callable returns a
+    # value equal to sentinel, the iteration will be terminated.

tests/run/cython_includes.pyx

@@ -23,6 +23,7 @@ cimport cpython.getargs
 cimport cpython.instance
 cimport cpython.int
 cimport cpython.iterator
+cimport cpython.iterobject
 cimport cpython.list
 cimport cpython.long
 cimport cpython.longintrepr