flake8: Ensure local check in hacking 2.0.0

local-check-factory support was dropped in hacking 2.0.0.
The release notes of hacking 2.0.0 [1] suggests to use flake8's
native local plugin support [2]. This commit follows it.

[1] https://docs.openstack.org/releasenotes/hacking/unreleased.html#relnotes-2-0-0
[2] https://flake8.pycqa.org/en/3.7.0/user/configuration.html#using-local-plugins

Change-Id: Ie19630a58d7b4ce735007b8abdd6ebac85d60946

horizon/hacking/checks.py

@@ -15,6 +15,8 @@
 
 import re
 
+from hacking import core
+
 """
 Guidelines for writing new hacking checks
  - Use only for Horizon specific tests. OpenStack general tests
@@ -31,11 +33,8 @@ Guidelines for writing new hacking checks
 mutable_default_args = re.compile(r"^\s*def .+\((.+=\{\}|.+=\[\])")
 
 
+@core.flake8ext
 def no_mutable_default_args(logical_line):
     msg = "M322: Method's default argument shouldn't be mutable!"
     if mutable_default_args.match(logical_line):
         yield (0, msg)
-
-
-def factory(register):
-    register(no_mutable_default_args)

tox.ini

@@ -184,8 +184,9 @@ max_line_length = 80
 import-order-style = pep8
 application-import-names = horizon,openstack_dashboard
 
-[hacking]
-local-check-factory = horizon.hacking.checks.factory
+[flake8:local-plugins]
+extension =
+  M322 = horizon.hacking.checks:no_mutable_default_args
 
 [doc8]
 # File extensions to check