gh-127873: Only check sys.flags.ignore_environment for PYTHON* env vars

[hugovk]

When -E is passed (and therefore sys.flags.ignore_environment is true), we should only ignore only env vars matching PYTHON*; in this case, only PYTHON_COLORS.

We should not ignore the other env vars: FORCE_COLOR/NO_COLOR/TERM.

Re: https://docs.python.org/3/using/cmdline.html#cmdoption-E

Fixing this revealed some tests that failed because of inserted colour codes. For example, in "ZeroDivisionError: division by zero", the parts "ZeroDivisionError", ":" and "division by zero" are all different colours.

For these test cases, I wrapped then with the @force_not_colorized decorator, or, where there were a number of them, I wrapped the test class in the new force_not_colorized_test_class decorator. This was a suggestion from #127223 (comment). And from that PR, I replaced some of the numerous function decorators with the class decorator.

• Issue: can_colorize() ignores FORCE_COLOR/NO_COLOR/TERM when -E is set #127873

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @hugovk for commit 80422ff 🤖

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[hugovk]

@pablogsal Please could you have a look at this? Thanks!