[3.5] fixed return type annotation for WSMessage.json() (#3720) (#3730)

(cherry picked from commit ad2c187)

Co-authored-by: fa1l <fa1l@users.noreply.github.com>

CHANGES/3720.bugfix

@@ -0,0 +1 @@
+Fixed return type annotation for WSMessage.json()

CONTRIBUTORS.txt

@@ -200,6 +200,7 @@ Simon Kennedy
 Sin-Woo Bang
 Stanislas Plum
 Stanislav Prokop
+Stepan Pletnev
 Stephen Granade
 Steven Seguin
 Sunghyun Hwang

aiohttp/http_websocket.py

@@ -83,7 +83,7 @@ class WSMsgType(IntEnum):
 class WSMessage(_WSMessageBase):
 
     def json(self, *,  # type: ignore
-             loads: Callable[[Any], Any]=json.loads) -> None:
+             loads: Callable[[Any], Any]=json.loads) -> Any:
         """Return parsed JSON data.
 
         .. versionadded:: 0.22