Fixed issue on Python 3.8

Author: agronholm

httpcore/_utils.py

@@ -15,7 +15,7 @@ def aclose(self) -> Awaitable[object]: ...
 
     _SupportsAcloseT = TypeVar("_SupportsAcloseT", bound=_SupportsAclose)
 
-    class aclosing(AbstractAsyncContextManager[_SupportsAcloseT, None]):
+    class aclosing(AbstractAsyncContextManager):
         def __init__(self, thing: _SupportsAcloseT) -> None:
             self.thing = thing