Workaround httpx deprecation warning in fastapi

tests/test_service.py

@@ -2,15 +2,19 @@
 from typing import Any
 
 import pytest
-from fastapi.testclient import TestClient
+from httpx import ASGITransport, AsyncClient
 
 from scanspec.service import PointsFormat, PointsRequest, app
 from scanspec.specs import Line
 
 
 @pytest.fixture
-def client() -> TestClient:
-    return TestClient(app)
+async def async_client():
+    """Fixture to create a FastAPI test client."""
+    async with AsyncClient(
+        transport=ASGITransport(app=app), base_url="http://test"
+    ) as async_test_client:
+        yield async_test_client
 
 
 # MIDPOINTS TEST(S) #