System tests/Python 3: syntax and imports (#9801)
* System tests: xmlrpclib and SimpleXMLRPCServer -> xmlrpc. Re #9798.
In Python 3, xmlrpclib and SimleXMLRPCServer are part of a unified xmlrpc module (the former is xmlrpc.client and the latter is xmlrpc.server). Thus combine the imports.
* System tests/library: use relative import.
* System tests: basestring -> str.
