Use union for python3.8 compatibility

f1tenth_gym/envs/track/cubic_spline.py

@@ -8,6 +8,7 @@
 import scipy.optimize as so
 from scipy import interpolate
 from numba import njit
+from typing import Union
 
 @njit(fastmath=False, cache=True)
 def nearest_point_on_trajectory(point, trajectory):
@@ -94,7 +95,7 @@ def __calc_s(self, x: np.ndarray, y: np.ndarray) -> np.ndarray:
         s.extend(np.cumsum(self.ds))
         return np.array(s)
 
-    def calc_position(self, s: float) -> tuple[float | None, float | None]:
+    def calc_position(self, s: float) -> tuple[Union[float, None], Union[float, None]]:
         """
         Calc position at the given s.