net-snmp: add optional python bindings.

- Rebased megahall's PR #24279 on homebrew/master
- Modified to use `which python` as requested there
net-snmp: add 5.7.2.1 bottle.

Closes #31685.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
net-snmp: pedantic cleanup.