Add cellobject.pxd (GH-3901)

The comments are taken from the C API documentation
(https://docs.python.org/3.9/c-api/cell.html).

Cython/Includes/cpython/cellobject.pxd

+from .object cimport PyObject
+
+cdef extern from "Python.h":
+
+    ############################################################################
+    # Cell Objects
+    ############################################################################
+
+    bint PyCell_Check(object ob)
+    # Return true if ob is a cell object; ob must not be NULL.
+
+    object PyCell_New(PyObject* ob)
+    # Return value: New reference.
+    # Create and return a new cell object containing the value ob. The
+    # parameter may be NULL.
+
+    object PyCell_Get(object cell)
+    # Return value: New reference.
+    # Return the contents of the cell object cell.
+
+    object PyCell_GET(object cell)
+    # Return value: Borrowed reference.
+    # Return the contents of the cell object cell, but without checking that
+    # cell is non-NULL and is a cell object.
+
+    int PyCell_Set(object cell, PyObject* value) except? -1
+    # Set the contents of the cell object cell to value. This releases the
+    # reference to any current content of the cell. value may be NULL. cell
+    # must be non-NULL; if it is not a cell object, -1 will be returned. On
+    # success, 0 will be returned.
+
+    void PyCell_SET(object cell, PyObject* value)
+    # Sets the value of the cell object cell to value. No reference counts are
+    # adjusted, and no checks are made for safety; cell must be non-NULL and
+    # must be a cell object.

tests/run/cython_includes.pyx

@@ -10,6 +10,7 @@ cimport cpython.bool
 cimport cpython.buffer
 cimport cpython.bytearray
 cimport cpython.bytes
+cimport cpython.cellobject
 cimport cpython.ceval
 cimport cpython.cobject
 cimport cpython.codecs