fix: use Union instead of |

Author: agoose77

myst_nb/core/config.py

@@ -1,7 +1,7 @@
 """Configuration for myst-nb."""
 import dataclasses as dc
 from enum import Enum
-from typing import Any, Callable, Iterable, Sequence
+from typing import Any, Callable, Iterable, Sequence, Union
 
 from myst_parser.config.dc_validators import (
     ValidatorType,
@@ -378,7 +378,7 @@ def __post_init__(self):
         },
         repr=False,
     )
-    mime_priority_overrides: Sequence[tuple[str, str, int | None]] = dc.field(
+    mime_priority_overrides: Sequence[tuple[str, str, Union[int, None]]] = dc.field(
         default=(),
         metadata={
             "validator": deep_iterable(