move misplaced PyObject_Format() C-API declaration to right .pxd file

Cython/Includes/cpython/buffer.pxd

@@ -103,6 +103,7 @@ cdef extern from "Python.h":
     # bytes” of the given length. Return 0 on success and -1 (with
     # raising an error) on error.
 
+    # DEPRECATED HERE: do not cimport from here, cimport from cpython.object instead
     object PyObject_Format(object obj, object format_spec)
     # Takes an arbitrary object and returns the result of calling
     # obj.__format__(format_spec).

Cython/Includes/cpython/object.pxd

@@ -285,3 +285,8 @@ cdef extern from "Python.h":
     # and returns NULL if the object cannot be iterated.
 
     Py_ssize_t Py_SIZE(object o)
+
+    object PyObject_Format(object obj, object format_spec)
+    # Takes an arbitrary object and returns the result of calling
+    # obj.__format__(format_spec).
+    # Added in Py2.6