starlette 1.10 TestClient: install httpx2; httpx still works but is deprecated

On starlette 1.10.0 (fastapi 0.162.0), importing fastapi.testclient or starlette.testclient in an environment with neither httpx2 nor httpx raises 'RuntimeError: The starlette.testclient module requires the httpx2 package to be installed'. A plain 'pip install fastapi' installs neither, because both are only in starlette's [full] extra. If only httpx (< 0.29) is installed, the import works but emits StarletteDeprecationWarning ('Using httpx with starlette.testclient is deprecated; install httpx2 instead'), which fails suites run with -W error. Add httpx2 to the test dependencies. The module does 'import httpx2 as httpx', so type hints and the response API are unchanged.