PEP 471: add the DirEntry.inode() method

Update the PEP according to the implementation.

pep-0471.txt

@@ -118,6 +118,11 @@ following attributes and methods:
   path) -- the equivalent of ``os.path.join(scandir_path,
   entry.name)``
 
+* ``inode()``: return the inode number of the entry. The result is cached on
+  the ``DirEntry`` object, use ``os.stat(entry.path,
+  follow_symlinks=False).st_ino`` to fetch up-to-date information.
+  On Unix, no system call is required.
+
 * ``is_dir(*, follow_symlinks=True)``: similar to
   ``pathlib.Path.is_dir()``, but the return value is cached on the
   ``DirEntry`` object; doesn't require a system call in most cases;