Add genobject.pxd

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

Cython/Includes/cpython/genobject.pxd

+from .pystate cimport PyFrameObject
+
+cdef extern from "Python.h":
+
+    ###########################################################################
+    # Generator Objects
+    ###########################################################################
+
+    bint PyGen_Check(object ob)
+    # Return true if ob is a generator object; ob must not be NULL.
+
+    bint PyGen_CheckExact(object ob)
+    # Return true if ob's type is PyGen_Type; ob must not be NULL.
+
+    object PyGen_New(PyFrameObject *frame)
+    # Return value: New reference.
+    # Create and return a new generator object based on the frame object. A
+    # reference to frame is stolen by this function. The argument must not be
+    # NULL.
+
+    object PyGen_NewWithQualName(PyFrameObject *frame, object name, object qualname)
+    # Return value: New reference.
+    # Create and return a new generator object based on the frame object, with
+    # __name__ and __qualname__ set to name and qualname. A reference to frame
+    # is stolen by this function. The frame argument must not be NULL.

tests/run/cython_includes.pyx

@@ -18,6 +18,7 @@ cimport cpython.dict
 cimport cpython.exc
 cimport cpython.float
 cimport cpython.function
+cimport cpython.genobject
 cimport cpython.getargs
 cimport cpython.instance
 cimport cpython.int