[WIP] bpo-37214: Add a new EncodingWarning warning category

[vstinner]

open() now emits a EncodingWarning if called with encoding=None.

https://bugs.python.org/issue37214

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@methane: What do you think of this approach? Does it make any sense to have an opt-in warning?

[methane]

@methane: What do you think of this approach? Does it make any sense to have an opt-in warning?

No. Warning everytime when encoding is omitted was @zooba 's suggestion. But he opposite to change default encoding strongly.
Without planned breaking change, adding warning which everyone ignore doesn't make sense to me.

In PEP 597 (2nd), I am proposing adding more option. macOS user can test their code easily with PYTHONTEXTENCODING=ascii. They can notice if their code is depending on default encoding is UTF-8.

Anyway, I'm thinking about advertising your UTF-8 mode.
Windows user can run code which assumes default text encoding is UTF-8.

[vstinner]

Ok. I may keep this PR open to show that always emit a warning can be a bad idea :-)

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I wrote this PR to discuss https://www.python.org/dev/peps/pep-0597/ but it's unrelated to the implementation of this PEP :-)

[vstinner]

I abandon this PR.