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.

[erlend-aasland]

This can go now :)

Suggested change

from collections.abc import Callable

[vstinner]

The import is unused.