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 4a7d84058d)

tests/sys/netpfil/common/pft_ping.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # SPDX-License-Identifier: BSD-2-Clause
 #

tests/sys/netpfil/pf/CVE-2019-5597.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # SPDX-License-Identifier: BSD-2-Clause-FreeBSD
 #

tests/sys/netpfil/pf/CVE-2019-5598.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # SPDX-License-Identifier: BSD-2-Clause-FreeBSD
 #