Improve warnings stubs (#3501)

* merge 2and3 for _warnings

* move warn and warn_explicit into _warnings

stdlib/3/_warnings.pyi

@@ -1,11 +0,0 @@
-from typing import Any, Dict, List, Optional, Tuple, Type
-
-_defaultaction: str
-_onceregistry: Dict[Any, Any]
-filters: List[Tuple[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: ...