Enable use of await for a class based view

[mattrasband]

Currently if you implement a middleware in the 3.5 syntax (native coroutines, e.g. async def and await) for class based views, they will yield a TypeError saying that that web.View is not awaitable. This is due to a new magic method __await__ for classes that needs to be implemented. This change simply uses the current __iter__ behavior (supporting 3.4 @asyncio.coroutine and yield from) and enables the 3.5 await keyword (required for native coroutines, e.g. async def)

A quick example of an error this can cause is in middleware written in the 3.5 syntax:

def db_middleware(dsn):
    async def factory(app, handler):
        async def middleware(request):
            db = app.get('db')
            if not db:
                app['db'] = db = await create_engine(dsn)
            request.app['db'] = db
            response = await handler(request)
        return middleware
    return factory

Without the __await__ method implemented a TypeError is raised.

Validated to work with 3.4+

[asvetlov]

Please add tests

[mattrasband]

@asvetlov Roger, I meant to ask - is there a good way to test python 3.4 vs 3.5 in the current setup? I saw that appveyor does it but wasn't sure the right way to try it locally?

[asvetlov]

Fixed by #717
@mrasband see the PR for test example.
Thanks for raising the issue

[mattrasband]

Awesome, you're the man. Sorry, I've been traveling a bit recently and haven't gotten back to doing it.

[lock]

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