net-snmp.rb: add optional python bindings to the formula

- Rebased megahall's PR #24279 on homebrew/master
- Modified to use `which python`

I didn't understand the 'python do' comments on the original PR;
I'm happy to make other changes if someone can point me to an
example to look at.
You'd have to ask the original author; I just ~~work~~ rebase here.