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/queue.pyi

@@ -0,0 +1,52 @@
+import sys
+from threading import Condition, Lock
+from typing import Any, Generic, Optional, TypeVar
+
+if sys.version_info >= (3, 9):
+    from types import GenericAlias
+
+_T = TypeVar("_T")
+
+class Empty(Exception): ...
+class Full(Exception): ...
+
+class Queue(Generic[_T]):
+    maxsize: int
+
+    mutex: Lock  # undocumented
+    not_empty: Condition  # undocumented
+    not_full: Condition  # undocumented
+    all_tasks_done: Condition  # undocumented
+    unfinished_tasks: int  # undocumented
+    queue: Any  # undocumented
+    def __init__(self, maxsize: int = ...) -> None: ...
+    def _init(self, maxsize: int) -> None: ...
+    def empty(self) -> bool: ...
+    def full(self) -> bool: ...
+    def get(self, block: bool = ..., timeout: Optional[float] = ...) -> _T: ...
+    def get_nowait(self) -> _T: ...
+    def _get(self) -> _T: ...
+    def put(self, item: _T, block: bool = ..., timeout: Optional[float] = ...) -> None: ...
+    def put_nowait(self, item: _T) -> None: ...
+    def _put(self, item: _T) -> None: ...
+    def join(self) -> None: ...
+    def qsize(self) -> int: ...
+    def _qsize(self) -> int: ...
+    def task_done(self) -> None: ...
+    if sys.version_info >= (3, 9):
+        def __class_getitem__(cls, item: Any) -> GenericAlias: ...
+
+class PriorityQueue(Queue[_T]): ...
+class LifoQueue(Queue[_T]): ...
+
+if sys.version_info >= (3, 7):
+    class SimpleQueue(Generic[_T]):
+        def __init__(self) -> None: ...
+        def empty(self) -> bool: ...
+        def get(self, block: bool = ..., timeout: Optional[float] = ...) -> _T: ...
+        def get_nowait(self) -> _T: ...
+        def put(self, item: _T, block: bool = ..., timeout: Optional[float] = ...) -> None: ...
+        def put_nowait(self, item: _T) -> None: ...
+        def qsize(self) -> int: ...
+        if sys.version_info >= (3, 9):
+            def __class_getitem__(cls, item: Any) -> GenericAlias: ...