tests: Require python3 when using Scapy

[jlduran]

Summary

python3 is a symbolic link that points to the current python 3.X
version.

It is possible for a system to have python (python 3.X) without the
python3 (symlink) package.

Test scripts that use Scapy are invoked using python3, so add it as a
required program.

Test Plan

Assuming the current python version is 3.11:

pkg install -y net/scapy
pkg remove -y python3
pkg install -y python310

The tests should skip, there should be no:

stderr:
env: python3: No such file or directory

[kp]

So this is correct, but it's a pretty marginal case that has to be deliberately provoked. I'm not sure it's worth the churn and ongoing maintenance burden to cope with it.

[asomers]

What maintenance burden, @kp? I think it's a worthwhile change. I've run into similar problems myself. I find it easy to have a system where something pulls in, say, lang/python311 but not lang/python3.