Using deprecated typing.Callable to maintain py3.8 compat

Author: matt-graham

s2fft/utils/iterative_refinement.py

@@ -1,7 +1,6 @@
 """Iterative scheme for improving accuracy of linear transforms."""
 
-from collections.abc import Callable
-from typing import TypeVar
+from typing import Callable, TypeVar
 
 T = TypeVar("T")