Replace custom deviceconfig serialization with mashumaru (#1274)

tests/conftest.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 import asyncio
 import sys
 import warnings
+from pathlib import Path
 from unittest.mock import MagicMock, patch
 
 import pytest
@@ -21,6 +22,13 @@ from .fixtureinfo import fixture_info  # noqa: F401
 turn_on = pytest.mark.parametrize("turn_on", [True, False])
 
 
+def load_fixture(foldername, filename):
+    """Load a fixture."""
+    path = Path(Path(__file__).parent / "fixtures" / foldername / filename)
+    with path.open() as fdp:
+        return fdp.read()
+
+
 async def handle_turn_on(dev, turn_on):
     if turn_on:
         await dev.turn_on()