LTS API: write an idiomatic "Hello WAMP"

[oberstet]

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[oberstet]

a first step towards #472

[oberstet]

How would a modern API for Python 3.5+ using async/await etc look like? What would be expected / least surprising / feel natural?

I have gone back to the drawing table starting from above assumption. I'll post here.

CALLER:

import asyncio
from autobahn.asyncio import wamp

url = 'ws://localhost:8080/ws'
realm = 'realm1'

async def go():
    async with wamp.connect(url) as transport:
        async with transport.join(realm) as session:
            result = await session.call('com.example.add2', 2, 3)
            print(result)

loop = asyncio.get_event_loop()
loop.run_until_complete(go())

[oberstet]

subseded/fixed in #964