In posix.uname(), decode nodename as ASCII/surrogateescape.

Also apply PEP 383 decoding to other fields.

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -486,8 +486,15 @@ process and user.
    :func:`socket.gethostname`  or even
    ``socket.gethostbyaddr(socket.gethostname())``.
 
+   The strings are converted using the file system encoding and the
+   ``'surrogateescape'`` error handler, except for ``nodename``, which
+   is converted as ASCII with the ``'surrogateescape'`` error handler
+   (see the :mod:`socket` module documentation for details).
+
    Availability: recent flavors of Unix.
 
+   .. versionchanged:: XXX
+      Previously, all the strings were converted as UTF-8.
 
 .. function:: unsetenv(key)
 
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -3046,12 +3046,13 @@ posix_uname(PyObject *self, PyObject *no
     Py_END_ALLOW_THREADS
     if (res < 0)
         return posix_error();
-    return Py_BuildValue("(sssss)",
-                         u.sysname,
-                         u.nodename,
-                         u.release,
-                         u.version,
-                         u.machine);
+    return Py_BuildValue("(NNNNN)",
+                         PyUnicode_DecodeFSDefault(u.sysname),
+                         PyUnicode_DecodeASCII(u.nodename, strlen(u.nodename),
+                                               "surrogateescape"),
+                         PyUnicode_DecodeFSDefault(u.release),
+                         PyUnicode_DecodeFSDefault(u.version),
+                         PyUnicode_DecodeFSDefault(u.machine));
 }
 #endif /* HAVE_UNAME */