Use MutableMapping for headers (#6969)

Author: q0w

stdlib/urllib/request.pyi

@@ -4,7 +4,7 @@ from _typeshed import StrOrBytesPath
 from email.message import Message
 from http.client import HTTPMessage, HTTPResponse, _HTTPConnectionProtocol
 from http.cookiejar import CookieJar
-from typing import IO, Any, Callable, ClassVar, Mapping, NoReturn, Pattern, Sequence, TypeVar, overload
+from typing import IO, Any, Callable, ClassVar, Mapping, MutableMapping, NoReturn, Pattern, Sequence, TypeVar, overload
 from urllib.error import HTTPError
 from urllib.response import addclosehook, addinfourl
 
@@ -52,7 +52,7 @@ class Request:
     origin_req_host: str
     selector: str
     data: bytes | None
-    headers: dict[str, str]
+    headers: MutableMapping[str, str]
     unredirected_hdrs: dict[str, str]
     unverifiable: bool
     method: str | None
@@ -61,7 +61,7 @@ class Request:
         self,
         url: str,
         data: bytes | None = ...,
-        headers: dict[str, str] = ...,
+        headers: MutableMapping[str, str] = ...,
         origin_req_host: str | None = ...,
         unverifiable: bool = ...,
         method: str | None = ...,