Fix comment that says cast is necessary for Python 3.5

The comment was correct when we were using json.loads. But since
switching to our own JSONDecoder instead, the decode() function only
supports str-type arguments.

Switch happened in #8106.

synapse/storage/_base.py

@@ -114,7 +114,7 @@ def db_to_json(db_content: Union[memoryview, bytes, bytearray, str]) -> Any:
         db_content = db_content.tobytes()
 
     # Decode it to a Unicode string before feeding it to the JSON decoder, since
-    # Python 3.5 does not support deserializing bytes.
+    # it only supports handling strings
     if isinstance(db_content, (bytes, bytearray)):
         db_content = db_content.decode("utf8")