Support python 3.13 in symbolic_formula

[joshua-cogliati-inl]

In python 3.13, how variables in exec are handled has changed (see PEP 667 https://peps.python.org/pep-0667/ ). This means that the lines:

                exec(f"x{ii} = sympy.Symbol('x_{ii}')")
                exec(f"x.append(x{ii})")

in symbolic_formula in MultKAN.py can error out with an error like:

>>> model.symbolic_formula()
Traceback (most recent call last):
  File "<python-input-45>", line 1, in <module>
    model.symbolic_formula()
    ~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/fred/pykan/kan/MultKAN.py", line 2277, in symbolic_formula
    exec(f"x.append(x{ii})")
    ~~~~^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 1, in <module>
NameError: name 'x1' is not defined

I suggest either using something like:

                new_var = sympy.Symbol(f'x_{ii}')
                x.append(new_var)

or

                new_var = f"x{ii}"
                sys._getframe().f_locals[new_var] = sympy.Symbol(f'x_{ii}')
                x.append(sys._getframe().f_locals[new_var])

instead of exec.