net-mgmt/netbox: Update dependencies after 6b168f112781

• Since release 0.18.0 of textproc/py-mkdocstrings, the handlers have been outsourced to separate modules and can be installed through the respective options.
  
  Because options PYTHON and PYTHONLEGACY of py-mkdocstrings are currently mutually exclusive and therefore non-default, explicitly define py-mkdocstrings-python-legacy as required dependency.

• Bump PORTREVISION due changed dependencies.