As discussed at the Epoxy PTG meeting, run an automated
upgrade tool to make code python 3.9+ compliant.

Result of running:

$ pyupgrade --py39-plus $(git ls-files | grep ".py$")

Fixed PEP8 errors introduced by pyupgrade by running:

$ autopep8 --select=E127,E128,E501 --max-line-length 79 -r \
  --in-place neutron_lib

Also did manual updates as necessary to fix other errors
and warnings after above commands. Removed a number of
pylint 'disable' directives.

Inspired by Octavia and Nova [0].

[0] https://review.opendev.org/c/openstack/nova/+/896986

Change-Id: Ic8c95898fc00d5463aa0992e844eee60ac3dc5a0