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/cmd.pyi

@@ -0,0 +1,39 @@
+from typing import IO, Any, Callable, List, Optional, Tuple
+
+class Cmd:
+    prompt: str
+    identchars: str
+    ruler: str
+    lastcmd: str
+    intro: Optional[Any]
+    doc_leader: str
+    doc_header: str
+    misc_header: str
+    undoc_header: str
+    nohelp: str
+    use_rawinput: bool
+    stdin: IO[str]
+    stdout: IO[str]
+    cmdqueue: List[str]
+    completekey: str
+    def __init__(self, completekey: str = ..., stdin: Optional[IO[str]] = ..., stdout: Optional[IO[str]] = ...) -> None: ...
+    old_completer: Optional[Callable[[str, int], Optional[str]]]
+    def cmdloop(self, intro: Optional[Any] = ...) -> None: ...
+    def precmd(self, line: str) -> str: ...
+    def postcmd(self, stop: bool, line: str) -> bool: ...
+    def preloop(self) -> None: ...
+    def postloop(self) -> None: ...
+    def parseline(self, line: str) -> Tuple[Optional[str], Optional[str], str]: ...
+    def onecmd(self, line: str) -> bool: ...
+    def emptyline(self) -> bool: ...
+    def default(self, line: str) -> bool: ...
+    def completedefault(self, *ignored: Any) -> List[str]: ...
+    def completenames(self, text: str, *ignored: Any) -> List[str]: ...
+    completion_matches: Optional[List[str]]
+    def complete(self, text: str, state: int) -> Optional[List[str]]: ...
+    def get_names(self) -> List[str]: ...
+    # Only the first element of args matters.
+    def complete_help(self, *args: Any) -> List[str]: ...
+    def do_help(self, arg: str) -> Optional[bool]: ...
+    def print_topics(self, header: str, cmds: Optional[List[str]], cmdlen: Any, maxcol: int) -> None: ...
+    def columnize(self, list: Optional[List[str]], displaywidth: int = ...) -> None: ...