tools: net: use python3 explicitly

The scripts require Python 3 and some distros are dropping
Python 2 support.
Reported-by: NStanislav Fomichev <sdf@google.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

tools/net/ynl/cli.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 
 import argparse

tools/net/ynl/ynl-gen-c.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import argparse
 import collections