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/copyreg.pyi

@@ -0,0 +1,16 @@
+from typing import Any, Callable, Hashable, List, Optional, SupportsInt, Tuple, TypeVar, Union
+
+_TypeT = TypeVar("_TypeT", bound=type)
+_Reduce = Union[Tuple[Callable[..., _TypeT], Tuple[Any, ...]], Tuple[Callable[..., _TypeT], Tuple[Any, ...], Optional[Any]]]
+
+__all__: List[str]
+
+def pickle(
+    ob_type: _TypeT,
+    pickle_function: Callable[[_TypeT], Union[str, _Reduce[_TypeT]]],
+    constructor_ob: Optional[Callable[[_Reduce[_TypeT]], _TypeT]] = ...,
+) -> None: ...
+def constructor(object: Callable[[_Reduce[_TypeT]], _TypeT]) -> None: ...
+def add_extension(module: Hashable, name: Hashable, code: SupportsInt) -> None: ...
+def remove_extension(module: Hashable, name: Hashable, code: int) -> None: ...
+def clear_extension_cache() -> None: ...