🐛 fix(config): read a set_env file as UTF-8 (#4059)

Author: MohammedAlkindi

docs/changelog/4059.bugfix.rst

@@ -0,0 +1,2 @@
+A ``set_env`` environment file (``file|.env``) is now read as UTF-8 instead of the platform's locale encoding, so
+non-ASCII values no longer arrive mangled on Windows - by :user:`MohammedAlkindi`.

src/tox/config/set_env.py

@@ -121,7 +121,7 @@ def _stream_env_file(self, filename: str, args: ConfigLoadArgs) -> Iterator[tupl
         if not env_file.exists():
             msg = f"{env_file} does not exist for set_env"
             raise Fail(msg)
-        for env_line in env_file.read_text().splitlines():
+        for env_line in env_file.read_text(encoding="utf-8").splitlines():
             env_line = env_line.strip()  # ruff:ignore[redefined-loop-name]
             if not env_line or env_line.startswith("#"):
                 continue

tests/config/test_set_env.py

@@ -201,6 +201,14 @@ def test_set_env_environment_file(
     }
 
 
+def test_set_env_environment_file_is_utf8(eval_set_env: EvalSetEnv) -> None:
+    set_env = eval_set_env(
+        "[testenv]\npackage=skip\nset_env=file|.env",
+        extra_files={".env": "GREETING=Grüße\n"},
+    )
+    assert set_env.load("GREETING") == "Grüße"
+
+
 @pytest.mark.parametrize(
     ("of_type", "config"),
     [