Change type hints to Union for py3.9 support

pybop/optimisers/scipy_optimisers.py

@@ -1,3 +1,5 @@
+from typing import Union
+
 import numpy as np
 from scipy.optimize import OptimizeResult, differential_evolution, minimize
 
@@ -175,7 +177,7 @@ def _run_optimiser(self):
 
         # Add callback storing history of parameter values
 
-        def base_callback(intermediate_result: OptimizeResult | np.ndarray):
+        def base_callback(intermediate_result: Union[OptimizeResult, np.ndarray]):
             """
             Log intermediate optimisation solutions. Depending on the
             optimisation algorithm, intermediate_result may be either