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/quopri.pyi

@@ -0,0 +1,6 @@
+from typing import BinaryIO
+
+def encode(input: BinaryIO, output: BinaryIO, quotetabs: int, header: int = ...) -> None: ...
+def encodestring(s: bytes, quotetabs: int = ..., header: int = ...) -> bytes: ...
+def decode(input: BinaryIO, output: BinaryIO, header: int = ...) -> None: ...
+def decodestring(s: bytes, header: int = ...) -> bytes: ...