use typevar for _item_or_default annotation

Otherwise you lose all typing when you use that because it returns Any.

xarray/conventions.py

@@ -2,7 +2,7 @@
 
 from collections import defaultdict
 from collections.abc import Hashable, Iterable, Mapping, MutableMapping
-from typing import TYPE_CHECKING, Any, Literal, Union
+from typing import TYPE_CHECKING, Any, Literal, Union, TypeVar
 
 import numpy as np
 import pandas as pd
@@ -383,8 +383,8 @@ def _update_bounds_encoding(variables: T_Variables) -> None:
                 if "calendar" in encoding:
                     bounds_encoding.setdefault("calendar", encoding["calendar"])
 
-
-def _item_or_default(obj: Mapping | Any, key: Hashable, default: Any = None):
+T = TypeVar("T")
+def _item_or_default(obj: Mapping[Any, T] | T, key: Hashable, default: T = None) -> T:
     """
     Return item by key if obj is mapping and key is present, else return default value.
     """