tests/sys/netpfil/pf: Add missing python3 requirements

This also fixes a typo in the dup test that caused the head function to
not be called. On my test system without python3 the tests are now
skipped instead of failing.

Reviewed By: kp
Differential Revision: https://reviews.freebsd.org/D28903

(cherry picked from commit 98202829d1b3727a8706f45d052fc9e9507b562b)