pyupgrade changes for Python3.9+

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

This patch is for pylint.

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

Also did manual updates as necessary to fix other errors
and warnings after above commands.

Inspired by Octavia and Nova [0].

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

Change-Id: Idc2bc19b018df6f74da0f4ed10c96abf6117720b

.pylintrc

@@ -37,7 +37,6 @@ disable=
  expression-not-assigned,
  fixme,
  global-statement,
- non-parent-init-called,
  not-callable,
  protected-access,
  redefined-builtin,
@@ -89,8 +88,6 @@ disable=
  too-many-return-statements,
  too-many-statements,
  consider-using-set-comprehension,
- useless-object-inheritance,
- super-with-arguments,
  use-dict-literal
 
 [BASIC]