Bug report

Bug description:

The terminal colour detection defaults to checking stderr's isatty, rather than stdout.

As suggested at #128317 (comment), let's default to checking stdout's isatty rather than stderr's.

Here some example output, plus when piping it:

Now	Proposal

CPython versions tested on:

3.13, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-128595: Default to stdout isatty for colour detection instead of stderr #128498
• gh-128595: Add test class helper to force no terminal colour #128687
• [3.13] gh-128595: Add test class helper to force no terminal colour (GH-128687) #128778
• [3.13] gh-128595: Default to stdout isatty for colour detection instead of stderr (GH-128498) #129057
• gh-128595: Fix test__colorize unexpected keyword argument 'file' on buildbots #129070