Improve pty.pyi (#6806)

Co-authored-by: Alex Waygood <[email protected]>

Author: sobolevn

stdlib/pty.pyi

@@ -1,16 +1,18 @@
 import sys
 from typing import Callable, Iterable
+from typing_extensions import Literal
 
 if sys.platform != "win32":
+    __all__ = ["openpty", "fork", "spawn"]
     _Reader = Callable[[int], bytes]
 
-    STDIN_FILENO: int
-    STDOUT_FILENO: int
-    STDERR_FILENO: int
+    STDIN_FILENO: Literal[0]
+    STDOUT_FILENO: Literal[1]
+    STDERR_FILENO: Literal[2]
 
-    CHILD: int
+    CHILD: Literal[0]
     def openpty() -> tuple[int, int]: ...
-    def master_open() -> tuple[int, str]: ...
-    def slave_open(tty_name: str) -> int: ...
+    def master_open() -> tuple[int, str]: ...  # deprecated, use openpty()
+    def slave_open(tty_name: str) -> int: ...  # deprecated, use openpty()
     def fork() -> tuple[int, int]: ...
     def spawn(argv: str | Iterable[str], master_read: _Reader = ..., stdin_read: _Reader = ...) -> int: ...