NO_COLOR follows no-color.org logic instead of tox boolean (#2727)

Co-authored-by: ptmcg <ptmcg@austin.rr.com>

docs/changelog/2719.feature.rst

@@ -0,0 +1,4 @@
+Modified handling of ``NO_COLOR`` environment variable, consistent with
+`de facto conventions <https://no-color.org>`_: any non-empty string will enable ``NO_COLOR`` (disable colorized
+output); no ``NO_COLOR`` variable or ``NO_COLOR`` with an empty string will disable ``NO_COLOR`` (enable colorized
+output) - by :user:`ptmcg`.

src/tox/config/cli/parser.py

@@ -290,7 +290,7 @@ def add_verbosity_flags(parser: ArgumentParser) -> None:
 
 def add_color_flags(parser: ArgumentParser) -> None:
     converter = StrConvert()
-    if converter.to_bool(os.environ.get("NO_COLOR", "")):
+    if os.environ.get("NO_COLOR", "") != "":
         color = "no"
     elif converter.to_bool(os.environ.get("FORCE_COLOR", "")):
         color = "yes"

tests/config/cli/test_parser.py

@@ -28,7 +28,7 @@ def test_parser_const_with_default_none(monkeypatch: MonkeyPatch) -> None:
 
 
 @pytest.mark.parametrize("is_atty", [True, False])
-@pytest.mark.parametrize("no_color", [None, "0", "1"])
+@pytest.mark.parametrize("no_color", [None, "0", "1", "", "\t", " ", "false", "true"])
 @pytest.mark.parametrize("force_color", [None, "0", "1"])
 @pytest.mark.parametrize("tox_color", [None, "bad", "no", "yes"])
 @pytest.mark.parametrize("term", [None, "xterm", "dumb"])
@@ -56,7 +56,7 @@ def test_parser_color(
 
     if tox_color in ("yes", "no"):
         expected = tox_color == "yes"
-    elif no_color == "1":
+    elif bool(no_color):
         expected = False
     elif force_color == "1":
         expected = True