Refactored sample code for Chapter 18 - "Concurrency with asyncio"

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

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NOTE: this directory contains the examples of chapter 18
rewritten using the new async/await syntax available from Python
3.5+, instead of the "yield-from" syntax of Python 3.3 and 3.4.
The code was tested with Python 3.7
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