Drop py36 and py37 support

We are not testing with py36 and py37 so we cannot be sure hacking works
with these versions.

Add releasenote that also mentions the previous changes in py35 and
flake8 support.

Sem-Ver: api-break
Change-Id: I798dd39afcf2f16a8660ab597b6f5bae70ab2a81

releasenotes/notes/drop-python36-834bb59e53d5fd71.yaml

@@ -0,0 +1,8 @@
+---
+upgrade:
+  - |
+    Support for Python 3.5 through 3.7 has been dropped. The minimum
+    supported Python version is now 3.8.
+  - |
+    ``hacking`` is now compatible with ``flake8~=4.0.1``. flake8 3.x
+    is no longer supported.

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 = >=3.6
+python_requires = >=3.8
 project_urls =
   Bug Tracker = https://bugs.launchpad.net/hacking
   Reviews = https://review.opendev.org/q/p:openstack/hacking+status:open
@@ -22,8 +22,6 @@ classifier =
     Operating System :: OS Independent
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9