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/asynchat.pyi

@@ -0,0 +1,39 @@
+import asyncore
+import socket
+import sys
+from abc import abstractmethod
+from typing import Optional, Sequence, Tuple, Union
+
+class simple_producer:
+    def __init__(self, data: bytes, buffer_size: int = ...) -> None: ...
+    def more(self) -> bytes: ...
+
+class async_chat(asyncore.dispatcher):
+    ac_in_buffer_size: int
+    ac_out_buffer_size: int
+    def __init__(self, sock: Optional[socket.socket] = ..., map: Optional[asyncore._maptype] = ...) -> None: ...
+    @abstractmethod
+    def collect_incoming_data(self, data: bytes) -> None: ...
+    @abstractmethod
+    def found_terminator(self) -> None: ...
+    def set_terminator(self, term: Union[bytes, int, None]) -> None: ...
+    def get_terminator(self) -> Union[bytes, int, None]: ...
+    def handle_read(self) -> None: ...
+    def handle_write(self) -> None: ...
+    def handle_close(self) -> None: ...
+    def push(self, data: bytes) -> None: ...
+    def push_with_producer(self, producer: simple_producer) -> None: ...
+    def readable(self) -> bool: ...
+    def writable(self) -> bool: ...
+    def close_when_done(self) -> None: ...
+    def initiate_send(self) -> None: ...
+    def discard_buffers(self) -> None: ...
+
+if sys.version_info < (3, 0):
+    class fifo:
+        def __init__(self, list: Sequence[Union[bytes, simple_producer]] = ...) -> None: ...
+        def __len__(self) -> int: ...
+        def is_empty(self) -> bool: ...
+        def first(self) -> bytes: ...
+        def push(self, data: Union[bytes, simple_producer]) -> None: ...
+        def pop(self) -> Tuple[int, bytes]: ...