Unfortunately networking-cisco[1] has not been updated for recent two
years, and it still supports only Python 2.7 and 3.5 according to its
setup.cfg file.
Let's deprecate support of the plugin since we don't expect any users
still using that outdated plugin with recent OpenStack releases.
[1] https://opendev.org/x/networking-cisco/
Change-Id: I8ccfd7ab762c66510e282ed29bcc8e8539997897