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/pty.pyi

@@ -0,0 +1,21 @@
+import sys
+from typing import Callable, Iterable, Tuple, Union
+
+_Reader = Callable[[int], bytes]
+
+STDIN_FILENO: int
+STDOUT_FILENO: int
+STDERR_FILENO: int
+
+CHILD: int
+
+def openpty() -> Tuple[int, int]: ...
+def master_open() -> Tuple[int, str]: ...
+def slave_open(tty_name: str) -> int: ...
+def fork() -> Tuple[int, int]: ...
+
+if sys.version_info >= (3, 4):
+    def spawn(argv: Union[str, Iterable[str]], master_read: _Reader = ..., stdin_read: _Reader = ...) -> int: ...
+
+else:
+    def spawn(argv: Union[str, Iterable[str]], master_read: _Reader = ..., stdin_read: _Reader = ...) -> None: ...