For backward compatibility with Python 3.3, Lib/asyncio/__init__.py exports asyncio.selectors and asyncio._overlapped symbols. Python 3.3 reached it's end of line last september, it's time to move on:
https://devguide.python.org/#status-of-python-branches

This change is backward incompatible.

Attached PR removes asyncio.selectors and asyncio._overlapped symbols, and use directly modules from the standard library. It documents the change in the "Porting to Python 3.7" section of the "What's New in Python 3.7" document.