Add support for flake8 4.x

flake 4 dropped support for py35 so this patch drops py35 support from
hacking too

Change-Id: Ia17d6978f4d7db776d986b75c3879cb9dc672e08
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

.zuul.yaml

@@ -2,7 +2,6 @@
     templates:
       - publish-openstack-docs-pti
       - openstack-cover-jobs
-      - openstack-python35-jobs
       - openstack-python3-jobs
       - release-notes-jobs-python3
     check:

requirements.txt

@@ -1 +1 @@
-flake8<3.9.0,>=3.8.0 # MIT
+flake8~=4.0.1 # MIT

setup.cfg

@@ -6,7 +6,7 @@ summary = OpenStack Hacking Guideline Enforcement
 description_file =
     README.rst
 home_page = https://docs.openstack.org/hacking/latest/
-python_requires = >=3.5
+python_requires = >=3.6
 project_urls =
   Bug Tracker = https://bugs.launchpad.net/hacking
   Reviews = https://review.opendev.org/q/p:openstack/hacking+status:open
@@ -22,7 +22,6 @@ classifier =
     Operating System :: OS Independent
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8