Remove default value from GridType

gprMax/updates/updates.py

@@ -18,13 +18,11 @@
 
 
 from abc import ABC, abstractmethod
-from typing import Generic
+from typing import Generic, TypeVar
-
-from typing_extensions import TypeVar
 
 from gprMax.grid.fdtd_grid import FDTDGrid
 
-GridType = TypeVar("GridType", bound=FDTDGrid, default=FDTDGrid)
+GridType = TypeVar("GridType", bound=FDTDGrid)
 
 
 class Updates(Generic[GridType], ABC):