sendProcessStdin() always raises INCORRECT_PARAMETERS on Python 3

[mnaberez]

Start supervisord in the foreground with a subprocess running:

$ python --version
Python 3.5.2

$ cat supervisord.conf
[supervisord]
loglevel = blather
nodaemon = true

[inet_http_server]
port = 127.0.0.1:9001

[supervisorctl]
serverurl = http://127.0.0.1:9001

[rpcinterface:supervisor]
supervisor.rpcinterface_factory = supervisor.rpcinterface:make_main_rpcinterface

[program:cat]
command = /bin/cat

$ supervisord -c supervisord.conf

In another terminal, try the XML-RPC method sendProcessStdin:

$ python
Python 3.5.2 (default, Sep 10 2016, 08:21:44)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from xmlrpc.client import ServerProxy
>>> server = ServerProxy('http://127.0.0.1:9001/RPC2', verbose=True)
>>> server.supervisor.sendProcessStdin('cat', 'hello')
send: b'POST /RPC2 HTTP/1.1\r\nHost: 127.0.0.1:9001\r\nAccept-Encoding: gzip\r\nContent-Type: text/xml\r\nUser-Agent: Python-xmlrpc/3.5\r\nContent-Length: 229\r\n\r\n'
send: b"<?xml version='1.0'?>\n<methodCall>\n<methodName>supervisor.sendProcessStdin</methodName>\n<params>\n<param>\n<value><string>cat</string></value>\n</param>\n<param>\n<value><string>hello</string></value>\n</param>\n</params>\n</methodCall>\n"
reply: 'HTTP/1.1 200 OK\r\n'
header: Content-Type header: Content-Length header: Date header: Server body: b"<?xml version='1.0'?>\n<methodResponse>\n<fault>\n<value><struct>\n<member>\n<name>faultString</name>\n<value><string>INCORRECT_PARAMETERS</string></value>\n</member>\n<member>\n<name>faultCode</name>\n<value><int>2</int></value>\n</member>\n</struct></value>\n</fault>\n</methodResponse>\n"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.5/xmlrpc/client.py", line 1092, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python3.5/xmlrpc/client.py", line 1432, in __request
    verbose=self.__verbose
  File "/usr/lib/python3.5/xmlrpc/client.py", line 1134, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python3.5/xmlrpc/client.py", line 1150, in single_request
    return self.parse_response(resp)
  File "/usr/lib/python3.5/xmlrpc/client.py", line 1322, in parse_response
    return u.close()
  File "/usr/lib/python3.5/xmlrpc/client.py", line 655, in close
    raise Fault(**self._stack[0])
xmlrpc.client.Fault: <Fault 2: 'INCORRECT_PARAMETERS'>

supervisord log output:

2016-10-09 10:09:10,394 TRAC 127.0.0.1:Medusa (V1.12) started at Sun Oct  9 10:09:10 2016
    Hostname: localhost
    Port:9001
2016-10-09 10:09:10,400 INFO RPC interface 'supervisor' initialized
2016-10-09 10:09:10,400 CRIT Server 'inet_http_server' running without any HTTP authentication checking
2016-10-09 10:09:10,401 INFO supervisord started with pid 10016
2016-10-09 10:09:11,404 INFO spawned: 'cat' with pid 10019
2016-10-09 10:09:12,408 INFO success: cat entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
2016-10-09 10:09:13,773 BLAT read event caused by <supervisor.http.supervisor_af_inet_http_server listening 127.0.0.1:9001 at 0x7f8684bf2128>
2016-10-09 10:09:13,774 BLAT read event caused by <supervisor.http.deferring_http_channel connected 127.0.0.1:37458 at 0x7f8684baf198 channel#: 0 requests:0>
2016-10-09 10:09:13,775 TRAC XML-RPC method called: supervisor.sendProcessStdin()
2016-10-09 10:09:13,775 TRAC XML-RPC method supervisor.sendProcessStdin() returned fault: [2] INCORRECT_PARAMETERS
2016-10-09 10:09:13,775 TRAC 127.0.0.1:37458 - - [09/Oct/2016:17:09:13 -0800] "POST /RPC2 HTTP/1.1" 200 397
2016-10-09 10:09:13,776 BLAT write event caused by <supervisor.http.deferring_http_channel connected 127.0.0.1:37458 at 0x7f8684baf198 channel#: 0 requests:1>

Tested on e73d017. The INCORRECT_PARAMETERS fault comes from the traverse function in xmlrpc.py catching a TypeError. It is the same exception seen #663 (comment). Once that is fixed, we should try the above again to be sure sendProcessStdin is also fixed.