Convert namedtuples to class syntax (#3321)

stdlib/2and3/sched.pyi

@@ -1,13 +1,12 @@
 import sys
 from typing import Any, Callable, Dict, List, NamedTuple, Optional, Text, Tuple
 
-Event = NamedTuple('Event', [
-    ('time', float),
-    ('priority', Any),
-    ('action', Callable[..., Any]),
-    ('argument', Tuple[Any, ...]),
-    ('kwargs', Dict[Text, Any]),
-])
+class Event(NamedTuple):
+    time: float
+    priority: Any
+    action: Callable[..., Any]
+    argument: Tuple[Any, ...]
+    kwargs: Dict[Text, Any]
 
 class scheduler:
     if sys.version_info >= (3, 3):