clean up

Author: MarkusSintonen

httpcore/_synchronization.py

@@ -45,6 +45,11 @@ def current_async_backend() -> AsyncBackend:
     if environment not in ("asyncio", "trio"):  # pragma: nocover
         raise RuntimeError("Running under an unsupported async environment.")
 
+    if environment == "asyncio" and anyio is None:  # pragma: nocover
+        raise RuntimeError(
+            "Running with asyncio requires installation of 'httpcore[asyncio]'."
+        )
+
     if environment == "trio" and trio is None:  # pragma: nocover
         raise RuntimeError(
             "Running with trio requires installation of 'httpcore[trio]'."