Fix blocking late import of httpcore from httpx (#141771)

There is a late import that blocks the event loop
in newer version
https://github.com/encode/httpx/blob/9e8ab40369bd3ec2cc8bff37ab79bf5769c8b00f/httpx/_transports/default.py#L75

homeassistant/helpers/httpx_client.py

@@ -7,6 +7,9 @@ import sys
 from types import TracebackType
 from typing import Any, Self
 
+# httpx dynamically imports httpcore, so we need to import it
+# to avoid it being imported later when the event loop is running
+import httpcore  # noqa: F401
 import httpx
 
 from homeassistant.const import APPLICATION_NAME, EVENT_HOMEASSISTANT_CLOSE, __version__