Proper interpretation of NO_COLOR environment variable

What's the problem this feature will solve?

tox interpretation of NO_COLOR uses StrConvert.to_bool() to look for specific string values (case insensitive match):

• 1, on, yes, true to "enable" NO_COLOR (i.e., disable colorizing output)
• 0, off, no, false, or "" to disable NO_COLOR (that is, enable colorizing output)

De facto conventions for NO_COLOR are to accept any non-blank string as an indication to disable colorizing output.

In addition, the StrConvert.to_bool() method raises a TypeError exception if some other value besides those listed above is found in the NO_COLOR environment variable. This will break pipeline builds for an essentially cosmetic reason.

Describe the solution you'd like

StrConvert.to_bool may be used in other places in tox to interpret various settings, so rather than modifying the logic in to_bool, I suggest modifying the code in (parser.py) add_color_flags() to just check for empty vs non-empty string when retrieving NO_COLOR from the environment, with a default of "".

def add_color_flags(parser: ArgumentParser) -> None:
    converter = StrConvert()
    # if converter.to_bool(os.environ.get("NO_COLOR", "")):   <-- CHANGE THIS LINE
    if os.environ.get("NO_COLOR", "") != "":
        color = "no"
    elif converter.to_bool(os.environ.get("FORCE_COLOR", "")):
        color = "yes"
    elif os.environ.get("TERM", "") == "dumb":
        color = "no"
    else:
        color = "yes" if sys.stdout.isatty() else "no"

    parser.add_argument(
        "--colored",
        default=color,
        choices=["yes", "no"],
        help="should output be enriched with colors, default is yes unless TERM=dumb or NO_COLOR is defined.",
    )

(Python does not expressly require the comparison against "", but doing so makes the purpose more explicit.)

Now pipeline scripts using NO_COLOR for other utilities that follow the "empty is off, non-empty is on" convention can also use tox without having to risk getting pipeline-breaking exceptions due to the value constraints in to_bool.

Alternative Solutions

Changing our current pipeline code to use NO_COLOR with one of the tox-compatible true values to disable colorizing, or leaving empty to enable colorizing is a workaround. But it is inconsistent with other conventions if one of the tox-compatible false values is set to enable colorizing, which by convention should disable it.

Additional context

None.

It seems worth mentioning that this behavior is new in 4.0. So there probably aren't a lot of existing scripts that depend on it.

Put in a PR with your change proposal.