Change type for urllib headers from Mapping to email.message.Message (#3345)

Also remove override of 'headers' in HTTPError

Closes #3344

stdlib/3/urllib/error.pyi

@@ -7,6 +7,5 @@ class URLError(IOError):
     reason: Union[str, BaseException]
 class HTTPError(URLError, addinfourl):
     code: int
-    headers: Dict[str, str]
     def __init__(self, url, code, msg, hdrs, fp) -> None: ...
 class ContentTooShortError(URLError): ...

stdlib/3/urllib/response.pyi

@@ -1,6 +1,7 @@
 # private module, we only expose what's needed
 
 from typing import BinaryIO, Iterable, List, Mapping, Optional, Type, TypeVar
+from email.message import Message
 from types import TracebackType
 
 _AIUT = TypeVar("_AIUT", bound=addbase)
@@ -31,8 +32,8 @@ class addbase(BinaryIO):
     def writelines(self, lines: Iterable[bytes]) -> None: ...
 
 class addinfo(addbase):
-    headers: Mapping[str, str]
-    def info(self) -> Mapping[str, str]: ...
+    headers: Message
+    def info(self) -> Message: ...
 
 class addinfourl(addinfo):
     url: str