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AIX provides sethostname() but it is not declared in any useful header. Fixed as follows:
diff -r 626a8e49f2a9 Modules/socketmodule.c
--- a/Modules/socketmodule.c Tue Jun 18 23:28:18 2013 +0200
+++ b/Modules/socketmodule.c Tue Jun 18 20:17:37 2013 -0700
@@ -4066,6 +4066,10 @@
 Py_buffer buf;
 int res, flag = 0;
 
+#ifdef _AIX
+extern int sethostname(const char *, size_t);
+#endif
+
 if (!PyArg_ParseTuple(args, "S:sethostname", &hnobj)) {
 PyErr_Clear();
 if (!PyArg_ParseTuple(args, "O&:sethostname",
It seemed best to declare it inside the function. I placed it below the other declarations, not immediately before the function use.

New changeset 9f8efcd78d0d by Christian Heimes in branch '3.3':
Issue #18259: Declare sethostname in socketmodule.c for AIX
http://hg.python.org/cpython/rev/9f8efcd78d0d
New changeset 14748397fc57 by Christian Heimes in branch 'default':
Issue #18259: Declare sethostname in socketmodule.c for AIX
http://hg.python.org/cpython/rev/14748397fc57 

Seems to work. I don't have root permission on the box so I can't actually test if the function call succeeds.
% ./python 
Python 3.4.0a0 (default:626a8e49f2a9, Jun 19 2013, 00:19:57) 
[GCC 4.7.1] on aix7
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> socket.sethostname('python')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
PermissionError: [Errno 1] Not owner