Skip test_xmlrpc_net when building python interpreters

[liyanhui1228]

Building python interpreter failed at test_xmlrpc_net, error message as below:

ERROR: test_python_builders (test.test_xmlrpc_net.PythonBuildersTest) Step #0: ---------------------------------------------------------------------- Step #0: Traceback (most recent call last): Step #0: File "/opt/sources/Python-3.5.4/Lib/test/test_xmlrpc_net.py", line 17, in test_python_builders Step #0: builders = server.getAllBuilders() Step #0: File "/opt/sources/Python-3.5.4/Lib/xmlrpc/client.py", line 1092, in __call__ Step #0: return self.__send(self.__name, args) Step #0: File "/opt/sources/Python-3.5.4/Lib/xmlrpc/client.py", line 1432, in __request Step #0: verbose=self.__verbose Step #0: File "/opt/sources/Python-3.5.4/Lib/xmlrpc/client.py", line 1134, in request Step #0: return self.single_request(host, handler, request_body, verbose) Step #0: File "/opt/sources/Python-3.5.4/Lib/xmlrpc/client.py", line 1167, in single_request Step #0: dict(resp.getheaders()) Step #0: xmlrpc.client.ProtocolError: <ProtocolError for buildbot.python.org/all/xmlrpc/: 503 Service Unavailable> 

Skip test_xmlrpc_net for now, to unblock releasing the new Python runtime image to address CVE.