PreparedRequests cast headers to native strings as well

https://github.com/psf/requests/blob/79f2ec3acc4e24fef6e6ce31ad7b1d4e2f77be31/requests/models.py#L493

stubs/requests/requests/models.pyi

@@ -74,7 +74,7 @@ class Request(RequestHooksMixin):
 class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
     method: str | None
     url: str | None
-    headers: CaseInsensitiveDict[str | bytes]
+    headers: CaseInsensitiveDict[str]
     body: bytes | str | None
     hooks: Any
     def __init__(self) -> None: ...