xmlrpc: properly close connection on SupervisorTransport.close

[perrinjerome]

Method on parent class ( xmlrpclib.Transport.close ) expect connection to be
self._connection[1], but SupervisorTransport use self.connection for the
socket. Implement SupervisorTransport.close which uses self.connection.

Fixes #1184

[perrinjerome]

for reference, ServerProxy.close calls Transport.close, which implementation is

• on python2.7 https://github.com/python/cpython/blob/2.7/Lib/xmlrpclib.py#L1396-L1400
• on pyton 3.7 https://github.com/python/cpython/blob/3.7/Lib/xmlrpc/client.py#L1251-L1255 ( it's same )