[Chapel Merge] Fix one more test to use python3

Branch: refs/heads/master
Revision: 35b0917
Author: mppf
Log Message:

Merge pull request #16626 from mppf/avect-python3

Fix one more test to use python3

Follow-up to PR #16560

Adjusts test/studies/amr/advection/level/Level_AdvectionCTU_driver.prediff
to use python3 rather than python. This one was missed in #16624.

Test change only - not reviewed.

Modified Files:
M test/studies/amr/advection/level/Level_AdvectionCTU_driver.prediff

Compare: https://github.com/chapel-lang/chapel/compare/f34cad5ab306...35b09173ab21