slepc4py: fix import error in ex13.py
Fixes a error occurring in one of the pipeline runners:
Traceback (most recent call last):
File "ex13.py", line 292, in <module>
main()
File "ex13.py", line 261, in main
evals, rg_params, ka, gt = solve(n)
File "ex13.py", line 239, in solve
expected_lam = scipy.optimize.newton(
AttributeError: module 'scipy' has no attribute 'optimize'