gh-148603: Update docs now UTF-8 is default (PEP 686)

[hugovk]

Fixes #148603.

Also update headers to sentence case in modified files:

https://devguide.python.org/documentation/style-guide/#capitalization

• Issue: open() documentation still says default encoding is locale-dependent in 3.15 #148603

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #34533562 | 📁 Comparing c3986bc against main (575fe39)

  🔍 Preview build  

15 files changed · + 1 added · ± 13 modified · - 1 deleted

+ Added

• library/tkinter.dialogs.html

± Modified

• c-api/bytes.html
• library/builtins.html
• library/csv.html
• library/functions.html
• library/intro.html
• library/io.html
• library/json.html
• reference/expressions.html
• tutorial/inputoutput.html
• using/windows.html
• and 3 more...

- Deleted

• library/dialog.html

[StanFromIreland]

This reads oddly.

[hugovk]

I've updated it, is it better now?

[vstinner]

LGTM. The changes LGTM, I didn't check if the change is exhaustive (update all references to open() default encoding).

I have a question for @methane about EncodingWarning, but it should not block this PR.

[vstinner]

@methane: It now looks wrong to emit EncodingWarning if open() is called with no encoding argument, since it defaults to UTF-8 which is fine, no? For me, the purpose of PEP 597 "Add optional EncodingWarning" is to detect when platform dependent locale encoding is used. Am I wrong?

At least, I see one advantage of the current behavior, it helps writing new code which is not platform dependent when running on older Python versions.

[methane]

EncodingWarning is a tool for people who want to identify all the places that will be affected by the transition to UTF-8 Mode and verify that the transition will not cause any problems.

Considering that some people may not start thinking about enabling UTF-8 Mode until Python 3.15, I think removing EncodingWarning immediately in Python 3.15 would be premature. How about removing it in Python 3.17 instead?

[vstinner]

Since -X warn_default_encoding is an opt-in option to emit EncodingWarning, I'm fine with emitting EncodingWarning on Python 3.15 when the encoding parameter is omitted. As I wrote, it should help writing new code on Python 3.15 which is compatible (behave as expected) on older Python versions.