Update encoding in pytest.py.

src/tox/pytest.py

@@ -161,7 +161,7 @@ def _setup_files(dest: Path, base: Path | None, content: dict[str, Any]) -> None
                 at_path.mkdir(exist_ok=True)
                 ToxProject._setup_files(at_path, None, value)
             elif isinstance(value, str):
-                at_path.write_text(textwrap.dedent(value))
+                at_path.write_text(textwrap.dedent(value), encoding="utf-8")
             elif value is None:
                 at_path.mkdir()
             else: