Fix return annotations of several methods that return self at runtime (#7070)

stdlib/asyncio/streams.pyi

@@ -1,5 +1,5 @@
 import sys
-from _typeshed import StrPath
+from _typeshed import Self, StrPath
 from typing import Any, AsyncIterator, Awaitable, Callable, Iterable, Optional
 
 from . import events, protocols, transports
@@ -117,7 +117,7 @@ class StreamWriter:
     def get_extra_info(self, name: str, default: Any = ...) -> Any: ...
     async def drain(self) -> None: ...
 
-class StreamReader:
+class StreamReader(AsyncIterator[bytes]):
     def __init__(self, limit: int = ..., loop: events.AbstractEventLoop | None = ...) -> None: ...
     def exception(self) -> Exception: ...
     def set_exception(self, exc: Exception) -> None: ...
@@ -129,5 +129,5 @@ class StreamReader:
     async def readuntil(self, separator: bytes = ...) -> bytes: ...
     async def read(self, n: int = ...) -> bytes: ...
     async def readexactly(self, n: int) -> bytes: ...
-    def __aiter__(self) -> AsyncIterator[bytes]: ...
+    def __aiter__(self: Self) -> Self: ...
     async def __anext__(self) -> bytes: ...