Always pass PY_COLORS to the environment

The environment variable is a popular way to force color output, an
example being pytest being run under a CI system.

docs/changelog/3171.feature.rst

@@ -0,0 +1 @@
+Always ``PY_COLORS`` to the environment

src/tox/tox_env/api.py

@@ -219,6 +219,7 @@ def _default_pass_env(self) -> list[str]:
             "LD_LIBRARY_PATH",  # location of libs
             "LDFLAGS",  # linker flags
             "HOME",  # needed for `os.path.expanduser()` on non-Windows systems
+            "PY_COLORS",  # Popular way to force color output in CI
         ]
         if sys.stdout.isatty():  # if we're on a interactive shell pass on the TERM
             env.append("TERM")

tests/session/cmd/test_show_config.py

@@ -130,6 +130,7 @@ def test_pass_env_config_default(tox_project: ToxProjectCreator, stdout_is_atty:
         + (["PROGRAMDATA"] if is_win else [])
         + (["PROGRAMFILES"] if is_win else [])
         + (["PROGRAMFILES(x86)"] if is_win else [])
+        + (["PY_COLORS"])
         + ["REQUESTS_CA_BUNDLE", "SSL_CERT_FILE"]
         + (["SYSTEMDRIVE", "SYSTEMROOT", "TEMP"] if is_win else [])
         + (["TERM"] if stdout_is_atty else [])