[BUG] snmptrapd logs error on every trap received

[lunkwill42]

Describe the bug

In NAV 5.5.1, snmptrapd has stopped working. Every received trap causes an exception+traceback to be logged.

It appears a syntax error somehow got into #2461 without any errors being triggered in the CI pipeline. This also needs to be looked at.

Expected behavior

Traps should be processed as before.

Screenshots

Tracebacks

snmptrapd.log is full of these:

Exception in _callback 'str' object is not callable
Traceback (most recent call last):
  File "/opt/venvs/nav/lib/python3.7/site-packages/pynetsnmp/netsnmp.py", line 464, in _callback
    sess.callback(pdu.contents)
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/snmptrapd/agent_pynetsnmp.py", line 221, in callback
    self._callback(addr, pdu)
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/snmptrapd/agent_pynetsnmp.py", line 130, in callback
    self._client_callback(trap)
  File "/usr/sbin/snmptrapd.py", line 234, in trap_handler
    _log_trap_handle_result(handled_by, trap)
  File "/usr/sbin/snmptrapd.py", line 239, in _log_trap_handle_result
    trap.netbox.sysname + ' ({})'.format(trap.agent) if trap.netbox else trap.agent
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/snmptrapd/trap.py", line 88, in netbox
    setattr(self, '_netbox', self._lookup_agent())
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/snmptrapd/trap.py", line 73, in _lookup_agent
    self.agent,
TypeError: 'str' object is not callable

Environment (please complete the following information):

• NAV 5.5.1
• Installed as Debian package

[lunkwill42]

Closed by #2498