Allow find/aggregate cursors to be used in async with-statements. Reported here: https://github.com/mongodb/motor/pull/172

When I use the following code:

async with self.coll.find() as cursor:
    async for record in cursor:
        yield record

It's going to throw an AttributeError.
So I added the asynchronous context manager method so that I did not have to close the cursor when I was done with it