The following Python code using the Qpid Proton library works with the Hono Sandbox.

from __future__ import print_function, unicode_literals
from proton import Message
from proton.handlers import MessagingHandler
from proton.reactor import Container

class HelloWorld(MessagingHandler):
    def __init__(self, server, address):
        super(HelloWorld, self).__init__()
        self.server = server
        self.address = address

    def on_start(self, event):
        print("connecting ...")
        conn = event.container.connect(self.server, sasl_enabled=True, allowed_mechs="PLAIN", allow_insecure_mechs=True, user="[email protected]", password="verysecret")
        event.container.create_sender(conn, self.address)

    def on_sendable(self, event):
        print("sending command")
        event.sender.send(Message(body="Hello World!", address="command/DEFAULT_TENANT/4711", content_type="text/plain", subject="call"))
        event.sender.close()
        event.connection.close()

Container(HelloWorld("amqp://hono.eclipseprojects.io:15672", "command/DEFAULT_TENANT")).run()

I guess in your code you are mainly missing to set the SASL properties. I also think that the username and password should better be set explicitly instead of as part of the URL because the username contains a @ character which also seems to be used in the URL as a delimiter.