pf tests: Explicitly ask for python3

If we install the scapy package (which we do list as a dependency) we
don't automatically install python (but we do have python3).

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC (“Netgate”’)

(cherry picked from commit 4a7d84058d88244c405fc0b73d6985681eb661f5)