Fix mypy errors for arithmetic analysis algorithms (#4053)

arithmetic_analysis/in_static_equilibrium.py

@@ -1,19 +1,14 @@
 """
 Checks if a system of forces is in static equilibrium.
-
-python/black : true
-flake8 : passed
-mypy : passed
 """
+from typing import List
 
-from __future__ import annotations
-
-from numpy import array, cos, cross, radians, sin  # type: ignore
+from numpy import array, cos, cross, radians, sin
 
 
 def polar_force(
     magnitude: float, angle: float, radian_mode: bool = False
-) -> list[float]:
+) -> List[float]:
     """
     Resolves force along rectangular components.
     (force, angle) => (force_x, force_y)