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/msvcrt.pyi

@@ -0,0 +1,24 @@
+import sys
+from typing import Text
+
+# This module is only available on Windows
+if sys.platform == "win32":
+    LK_LOCK: int
+    LK_NBLCK: int
+    LK_NBRLCK: int
+    LK_RLCK: int
+    LK_UNLCK: int
+    def locking(__fd: int, __mode: int, __nbytes: int) -> None: ...
+    def setmode(__fd: int, __mode: int) -> int: ...
+    def open_osfhandle(__handle: int, __flags: int) -> int: ...
+    def get_osfhandle(__fd: int) -> int: ...
+    def kbhit() -> bool: ...
+    def getch() -> bytes: ...
+    def getwch() -> Text: ...
+    def getche() -> bytes: ...
+    def getwche() -> Text: ...
+    def putch(__char: bytes) -> None: ...
+    def putwch(__unicode_char: Text) -> None: ...
+    def ungetch(__char: bytes) -> None: ...
+    def ungetwch(__unicode_char: Text) -> None: ...
+    def heapmin() -> None: ...