pf tests: Add python3 as a dependency

These tests use pft_ping.py, which requires python3 and Scapy.

Reviewed by: ngie
Approved by: emaste (mentor)
Fixes: 65c318630123 ("pf: Add prefer-ipv6-nexthop option for route-to pools")
MFC after: 10 days
Differential Revision: https://reviews.freebsd.org/D52589

(cherry picked from commit dea1dd66905462e3b65688928dc4016eeae4ce80)