Updated sample code for Chapter 17 - "Concurrency with futures"

From the book "Fluent Python" by Luciano Ramalho (O'Reilly, 2015) http://shop.oreilly.com/product/0636920032519.do

This directory contains code updated to run with Python 3.7 and aiohttp 3.5. When the first edition of "Fluent Python" was written, the asyncio package was provisional, and the latest version of aiohttp was 0.13.1. The API for both packages had significant breaking changes.