io.FileIO and io.open should support openat

Modules/_io/_iomodule.c

 /*
     An implementation of the new I/O lib as defined by PEP 3116 - "New I/O"
     
     Classes defined here: UnsupportedOperation, BlockingIOError.
     Functions defined here: open().
     
     Mostly written by Amaury Forgeot d'Arc
 */
 
 #define PY_SSIZE_T_CLEAN
@@ skipping 77 matching lines @@
 "   I/O classes. open() uses the file's blksize (as obtained by os.stat) if\n"
 "   possible.\n"
     );
 
 
 /*
  * The main open() function
  */
 PyDoc_STRVAR(open_doc,
 "open(file, mode='r', buffering=-1, encoding=None,\n"
-"     errors=None, newline=None, closefd=True) -> file object\n"
+"     errors=None, newline=None, closefd=True, opener=None) -> file object\n"
 "\n"
 "Open file and return a stream.  Raise IOError upon failure.\n"
 "\n"
 "file is either a text or byte string giving the name (and the path\n"
 "if the file isn't in the current working directory) of the file to\n"
 "be opened or an integer file descriptor of the file to be\n"
 "wrapped. (If a file descriptor is given, it is closed when the\n"
 "returned I/O object is closed, unless closefd is set to False.)\n"
 "\n"
 "mode is an optional string that specifies the mode in which the file\n"
@@ skipping 73 matching lines @@
 "\n"
 "* On output, if newline is None, any '\\n' characters written are\n"
 "  translated to the system default line separator, os.linesep. If\n"
 "  newline is '', no translation takes place. If newline is any of the\n"
 "  other legal values, any '\\n' characters written are translated to\n"
 "  the given string.\n"
 "\n"
 "If closefd is False, the underlying file descriptor will be kept open\n"
 "when the file is closed. This does not work when a file name is given\n"
 "and must be True in that case.\n"
+"\n"
+"A custom opener can be used by passing a callable as *opener*. The\n"
+"underlying file descriptor for the file object is then obtained by\n"
+"calling *opener* with (*file*, *flags*). *opener* must return an open\n"
+"file descriptor (passing os.open as *opener* results in functionality\n"
+"similar to passing None).\n"
 "\n"
 "open() returns a file object whose type depends on the mode, and\n"
 "through which the standard file operations such as reading and writing\n"
 "are performed. When open() is used to open a file in a text mode ('w',\n"
 "'r', 'wt', 'rt', etc.), it returns a TextIOWrapper. When used to open\n"
 "a file in a binary mode, the returned class varies: in read binary\n"
 "mode, it returns a BufferedReader; in write binary and append binary\n"
 "modes, it returns a BufferedWriter, and in read/write mode, it returns\n"
 "a BufferedRandom.\n"
 "\n"
@@ skipping 521 matching lines @@
     state->initialized = 1;
 
     return m;
 
   fail:
     Py_XDECREF(state->os_module);
     Py_XDECREF(state->unsupported_operation);
     Py_DECREF(m);
     return NULL;
 }