Depend on useful-types only when type-checking

pymbolic/mapper/substitutor.py

@@ -39,15 +39,17 @@
 THE SOFTWARE.
 """
 from collections.abc import Callable
-from typing import Any
-
-from useful_types import SupportsGetItem, SupportsItems
+from typing import TYPE_CHECKING, Any
 
 from pymbolic.mapper import CachedIdentityMapper, IdentityMapper
 from pymbolic.primitives import AlgebraicLeaf
 from pymbolic.typing import ExpressionT
 
 
+if TYPE_CHECKING:
+    from useful_types import SupportsGetItem, SupportsItems
+
+
 class SubstitutionMapper(IdentityMapper[[]]):
     def __init__(
         self, subst_func: Callable[[AlgebraicLeaf], ExpressionT | None]