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/uu.pyi

@@ -0,0 +1,16 @@
+import sys
+from typing import BinaryIO, Optional, Text, Union
+
+_File = Union[Text, BinaryIO]
+
+class Error(Exception): ...
+
+if sys.version_info >= (3, 7):
+    def encode(
+        in_file: _File, out_file: _File, name: Optional[str] = ..., mode: Optional[int] = ..., *, backtick: bool = ...
+    ) -> None: ...
+
+else:
+    def encode(in_file: _File, out_file: _File, name: Optional[str] = ..., mode: Optional[int] = ...) -> None: ...
+
+def decode(in_file: _File, out_file: Optional[_File] = ..., mode: Optional[int] = ..., quiet: int = ...) -> None: ...