Improve warnings stubs (#3501)

* merge 2and3 for _warnings

* move warn and warn_explicit into _warnings

stdlib/2/_warnings.pyi

@@ -1,16 +0,0 @@
-from typing import Any, Dict, List, Optional, Tuple, Type
-
-default_action: str
-filters: List[Tuple[Any, ...]]
-once_registry: Dict[Any, Any]
-
-def warn(message: Warning, category: Optional[Type[Warning]] = ..., stacklevel: int = ...) -> None: ...
-def warn_explicit(
-    message: Warning,
-    category: Optional[Type[Warning]],
-    filename: str,
-    lineno: int,
-    module: Any = ...,
-    registry: Dict[Any, Any] = ...,
-    module_globals: Dict[Any, Any] = ...,
-) -> None: ...