gh-128595: Add test class helper to force no terminal colour

[hugovk]

Split out from #128498, as requested at #128498 (comment).

• Issue: Default to stdout isatty for colour detection instead of stderr #128595

[serhiy-storchaka]

• You can disable colorizing for the whole class in setUpClass() instead of setUp().
• Would not it be simpler to implement the core functionality as a generator-based context manager? You can use enterContext() or enterClassContext() with it.
• You can use test.support.os_helper.EnvironmentVarGuard() to restore the environment and test.support.swap_attr() to restore _colorize.can_colorize.

You can also implement this as a mixin instead of patching a method (use a super() call in an overridden method). I do not say that it would be better, but it is just an alternative which you could have overlooked.

[vstinner]

Oh nice, the new code is more readable, I prefer context managers :-)

[serhiy-storchaka]

LGTM. Much clearer now!

[miss-islington-app]

Sorry, @hugovk, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker afb9dc887c6e8ae17b6a54c6124399e8bdc82253 3.13 

[hugovk]

Thanks for the reviews!

[bedevere-app]

GH-128778 is a backport of this pull request to the 3.13 branch.