[v9_16] Fix "forward" system test requirements

Commit 59d1eb3f added a Python-based name server (bin/tests/system/forward/ans11/ans.py) to the "forward" system test, but did not update the bin/tests/system/forward/prereq.sh script to ensure Python is present in the test environment before the "forward" system test is run. Update bin/tests/system/forward/prereq.sh to enforce that requirement.

Test pipeline which includes Windows jobs:

https://gitlab.isc.org/isc-projects/bind9/-/pipelines/101733

See !6117 (merged) for the BIND 9.18+ counterpart of this MR.