[v1] Remove import try / except block

asciinema/asciicast/v1.py

@@ -1,14 +1,10 @@
 import json
 from codecs import StreamReader
+from json.decoder import JSONDecodeError
 from typing import Any, Dict, Generator, List, Optional, TextIO, Union
 
 from .events import to_absolute_time
 
-try:
-    from json.decoder import JSONDecodeError
-except ImportError:
-    JSONDecodeError = ValueError  # type: ignore
-
 
 class LoadError(Exception):
     pass