Removed imported symbols that are not accessed or re-exported (#4387)

Co-authored-by: Eric Traut <erictr@microsoft.com>

stdlib/3/urllib/error.pyi

@@ -1,4 +1,4 @@
-from typing import Dict, Union
+from typing import Union
 from urllib.response import addinfourl
 
 # Stubs for urllib.error

stdlib/3/urllib/response.pyi

@@ -2,7 +2,7 @@
 
 from email.message import Message
 from types import TracebackType
-from typing import IO, Any, BinaryIO, Callable, Iterable, List, Mapping, Optional, Tuple, Type, TypeVar
+from typing import IO, Any, BinaryIO, Callable, Iterable, List, Optional, Tuple, Type, TypeVar
 
 _AIUT = TypeVar("_AIUT", bound=addbase)