Fix stubs that assumed reexport without as

builtins/3/_codecs.pyi

@@ -1,6 +1,6 @@
 """Stub file for the '_codecs' module."""
 
-from typing import Any, AnyStr, Callable, Tuple, Optional
+from typing import Any, AnyStr, Callable, Tuple, Optional, Dict
 
 import codecs
 

builtins/3/_warnings.pyi

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, List
 
 _defaultaction = ...  # type: str
 _onceregistry = ...  # type: dict

builtins/3/signal.pyi

@@ -1,6 +1,6 @@
 """Stub file for the 'signal' module."""
 
-from typing import Any, Callable, List, Tuple, Dict, Generic, Union, Optional, Iterable
+from typing import Any, Callable, List, Tuple, Dict, Generic, Union, Optional, Iterable, Set
 from types import FrameType
 
 class ItimerError(IOError): ...