Issue 32154: asyncio: Don't export selectors and _overlapped in asyncio namespace

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.

New changeset 4271dfd7815c05fd39b515c240050b3585bdfcd4 by Victor Stinner in branch 'master':
bpo-32154: Remove asyncio.selectors (#4605)
https://github.com/python/cpython/commit/4271dfd7815c05fd39b515c240050b3585bdfcd4

I also proposed the PR 4609 to remove the old asyncio.windows_utils.socketpair alias.

Is this function publicly documented? If this is true, it should first be deprecated.

It is left in the __all__ list.

Serhiy Storchaka: "Is this function publicly documented? If this is true, it should first be deprecated."

It's not documented, but I found a reference in the documentation:
---
try:
    from socket import socketpair
except ImportError:
    from asyncio.windows_utils import socketpair
---

This code continues to work on Python 3.5 and newer, the "except" block will just never be taken.


> It is left in the __all__ list.

Oops, fixed in my PR.

New changeset ac577d7d0bd27a69921ced14c09172235ceebab5 by Victor Stinner in branch 'master':
bpo-32154: Remove asyncio.windows_utils.socketpair (#4609)
https://github.com/python/cpython/commit/ac577d7d0bd27a69921ced14c09172235ceebab5