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/tty.pyi

@@ -0,0 +1,15 @@
+from typing import IO, Union
+
+_FD = Union[int, IO[str]]
+
+# XXX: Undocumented integer constants
+IFLAG: int
+OFLAG: int
+CFLAG: int
+LFLAG: int
+ISPEED: int
+OSPEED: int
+CC: int
+
+def setraw(fd: _FD, when: int = ...) -> None: ...
+def setcbreak(fd: _FD, when: int = ...) -> None: ...