Use TypeAlias for type aliases where possible, part II (#7667)

stdlib/selectors.pyi

@@ -3,8 +3,9 @@ from _typeshed import FileDescriptor, FileDescriptorLike, Self
 from abc import ABCMeta, abstractmethod
 from collections.abc import Mapping
 from typing import Any, NamedTuple
+from typing_extensions import TypeAlias
 
-_EventMask = int
+_EventMask: TypeAlias = int
 
 EVENT_READ: _EventMask
 EVENT_WRITE: _EventMask