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/chunk.pyi

@@ -0,0 +1,20 @@
+from typing import IO
+
+class Chunk:
+    closed: bool
+    align: bool
+    file: IO[bytes]
+    chunkname: bytes
+    chunksize: int
+    size_read: int
+    offset: int
+    seekable: bool
+    def __init__(self, file: IO[bytes], align: bool = ..., bigendian: bool = ..., inclheader: bool = ...) -> None: ...
+    def getname(self) -> bytes: ...
+    def getsize(self) -> int: ...
+    def close(self) -> None: ...
+    def isatty(self) -> bool: ...
+    def seek(self, pos: int, whence: int = ...) -> None: ...
+    def tell(self) -> int: ...
+    def read(self, size: int = ...) -> bytes: ...
+    def skip(self) -> None: ...