Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/sched.pyi

@@ -0,0 +1,39 @@
+import sys
+from typing import Any, Callable, Dict, List, NamedTuple, Optional, Text, Tuple
+
+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):
+        def __init__(self, timefunc: Callable[[], float] = ..., delayfunc: Callable[[float], None] = ...) -> None: ...
+        def enterabs(
+            self,
+            time: float,
+            priority: Any,
+            action: Callable[..., Any],
+            argument: Tuple[Any, ...] = ...,
+            kwargs: Dict[str, Any] = ...,
+        ) -> Event: ...
+        def enter(
+            self,
+            delay: float,
+            priority: Any,
+            action: Callable[..., Any],
+            argument: Tuple[Any, ...] = ...,
+            kwargs: Dict[str, Any] = ...,
+        ) -> Event: ...
+        def run(self, blocking: bool = ...) -> Optional[float]: ...
+    else:
+        def __init__(self, timefunc: Callable[[], float], delayfunc: Callable[[float], None]) -> None: ...
+        def enterabs(self, time: float, priority: Any, action: Callable[..., Any], argument: Tuple[Any, ...]) -> Event: ...
+        def enter(self, delay: float, priority: Any, action: Callable[..., Any], argument: Tuple[Any, ...]) -> Event: ...
+        def run(self) -> None: ...
+    def cancel(self, event: Event) -> None: ...
+    def empty(self) -> bool: ...
+    @property
+    def queue(self) -> List[Event]: ...