py37 compat

Author: AlexWaygood

stdlib/asyncio/unix_events.pyi

@@ -60,7 +60,9 @@ if sys.platform != "win32":
     class BaseChildWatcher(AbstractChildWatcher, metaclass=ABCMeta):
         def __init__(self) -> None: ...
         def close(self) -> None: ...
-        def is_active(self) -> bool: ...
+        if sys.version_info >= (3, 8):
+            def is_active(self) -> bool: ...
+
         def attach_loop(self, loop: AbstractEventLoop | None) -> None: ...
 
     class SafeChildWatcher(BaseChildWatcher):