xmlrpc: Enable 'allow_none'

This is a custom extension to the XML-RPC spec so it's not enabled by
default. In truth, the server shouldn't be returning `None` values but
there's a bug somewhere. The server is frozen so we need to fix this
here, on the client side.

While we're here, 'Server' in the 'xmlrpc.client' library is a legacy
alias for 'ServerProxy'. Update the call.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #22

Author: stephenfin

pwclient/api.py

@@ -154,7 +154,11 @@ def __init__(self, server, *, username=None, password=None, token=None):
             transport.set_credentials(username, password)
 
         try:
-            rpc = xmlrpc.xmlrpclib.Server(self._server, transport=transport)
+            rpc = xmlrpc.xmlrpclib.ServerProxy(
+                self._server,
+                transport=transport,
+                allow_none=True,
+            )
         except (IOError, OSError):
             raise exceptions.APIError(f'Unable to connect to {self._server}')