[docs] [issue33666] Document removal of os.errno

[Petr Viktorin]

Petr Viktorin <encukou at gmail.com> added the comment:

> Even if it is documented, arn't people know it by running their code on Python 3.7?  How the document help them?

"What's new" is the right place to check if something breaks for you on Python 3.7. Let's make it useful.

> It's very easy to know `errno` module when find ImportError.  And it's much easier than checking "waht's new" document.  So I doubt it's worth.

I disagree. Sure, some people will find answers on Stack Overflow or blog posts, but all those should link to official docs.

> And if people see the document, people may think "all removed subimports should be documented" although only os.errno is special.
> If document os.errno removal, please note about all undocumented subimports are implementation detail and will be removed without any timing, even on micro version.  (We will remove subimports for various reasons; avoiding huge unnecessary dependency, fixing regression caused by circular imports, etc...)

+1. Let's make that the main point. Something like the text below?

Several undocumented internal imports were removed. One example is that `os.errno` is no longer available; use `import errno` directly instead.
Note that such undocumented internal imports may be removed any time without notice, even in micro version releases.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33666>
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