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/sndhdr.pyi

@@ -0,0 +1,17 @@
+import sys
+from _typeshed import AnyPath
+from typing import NamedTuple, Optional, Tuple, Union
+
+if sys.version_info >= (3, 5):
+    class SndHeaders(NamedTuple):
+        filetype: str
+        framerate: int
+        nchannels: int
+        nframes: int
+        sampwidth: Union[int, str]
+    _SndHeaders = SndHeaders
+else:
+    _SndHeaders = Tuple[str, int, int, int, Union[int, str]]
+
+def what(filename: AnyPath) -> Optional[_SndHeaders]: ...
+def whathdr(filename: AnyPath) -> Optional[_SndHeaders]: ...