Move Request.headers back to Mapping (#7441)

Author: nateprewitt

src/requests/_types.py

@@ -109,7 +109,7 @@ class _ValidatedRequest(PreparedRequest):
         bytes | str | Iterable[bytes | str] | SupportsRead[bytes | str] | None
     )
 
-    HeadersType: TypeAlias = MutableMapping[str, str | bytes] | None
+    HeadersType: TypeAlias = Mapping[str, str | bytes] | None
 
     CookiesType: TypeAlias = RequestsCookieJar | Mapping[str, str]
 

src/requests/models.py

@@ -83,7 +83,6 @@
 )
 
 if TYPE_CHECKING:
-    from collections.abc import MutableMapping
     from http.cookiejar import CookieJar
 
     from typing_extensions import Self
@@ -311,7 +310,7 @@ class Request(RequestHooksMixin):
 
     method: str | None
     url: _t.UriType | None
-    headers: MutableMapping[str, str | bytes]
+    headers: Mapping[str, str | bytes]
     files: _t.FilesType
     data: _t.DataType
     json: _t.JsonType