Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/@python2/io.pyi

@@ -0,0 +1,40 @@
+from typing import IO, Any, Union
+
+import _io
+from _io import (
+    DEFAULT_BUFFER_SIZE as DEFAULT_BUFFER_SIZE,
+    BlockingIOError as BlockingIOError,
+    BufferedRandom as BufferedRandom,
+    BufferedReader as BufferedReader,
+    BufferedRWPair as BufferedRWPair,
+    BufferedWriter as BufferedWriter,
+    BytesIO as BytesIO,
+    FileIO as FileIO,
+    IncrementalNewlineDecoder as IncrementalNewlineDecoder,
+    StringIO as StringIO,
+    TextIOWrapper as TextIOWrapper,
+    UnsupportedOperation as UnsupportedOperation,
+    open as open,
+)
+
+def _OpenWrapper(
+    file: Union[str, unicode, int],
+    mode: unicode = ...,
+    buffering: int = ...,
+    encoding: unicode = ...,
+    errors: unicode = ...,
+    newline: unicode = ...,
+    closefd: bool = ...,
+) -> IO[Any]: ...
+
+SEEK_SET: int
+SEEK_CUR: int
+SEEK_END: int
+
+class IOBase(_io._IOBase): ...
+class RawIOBase(_io._RawIOBase, IOBase): ...
+class BufferedIOBase(_io._BufferedIOBase, IOBase): ...
+
+# Note: In the actual io.py, TextIOBase subclasses IOBase.
+# (Which we don't do here because we don't want to subclass both TextIO and BinaryIO.)
+class TextIOBase(_io._TextIOBase): ...