add missing attributes in codecs module (#13145)

stdlib/codecs.pyi

@@ -3,7 +3,7 @@ from _codecs import *
 from _typeshed import ReadableBuffer
 from abc import abstractmethod
 from collections.abc import Callable, Generator, Iterable
-from typing import Any, BinaryIO, Final, Literal, Protocol, TextIO
+from typing import Any, BinaryIO, ClassVar, Final, Literal, Protocol, TextIO
 from typing_extensions import Self
 
 __all__ = [
@@ -202,6 +202,7 @@ class StreamWriter(Codec):
     def write(self, object: str) -> None: ...
     def writelines(self, list: Iterable[str]) -> None: ...
     def reset(self) -> None: ...
+    def seek(self, offset: int, whence: int = 0) -> None: ...
     def __enter__(self) -> Self: ...
     def __exit__(self, type: type[BaseException] | None, value: BaseException | None, tb: types.TracebackType | None) -> None: ...
     def __getattr__(self, name: str, getattr: Callable[[Any, str], Any] = ...) -> Any: ...
@@ -209,11 +210,14 @@ class StreamWriter(Codec):
 class StreamReader(Codec):
     stream: _ReadableStream
     errors: str
+    # This is set to str, but some subclasses set to bytes instead.
+    charbuffertype: ClassVar[type] = ...
     def __init__(self, stream: _ReadableStream, errors: str = "strict") -> None: ...
     def read(self, size: int = -1, chars: int = -1, firstline: bool = False) -> str: ...
     def readline(self, size: int | None = None, keepends: bool = True) -> str: ...
     def readlines(self, sizehint: int | None = None, keepends: bool = True) -> list[str]: ...
     def reset(self) -> None: ...
+    def seek(self, offset: int, whence: int = 0) -> None: ...
     def __enter__(self) -> Self: ...
     def __exit__(self, type: type[BaseException] | None, value: BaseException | None, tb: types.TracebackType | None) -> None: ...
     def __iter__(self) -> Self: ...