Drop py3.6 and py3.7 from devstack-tools

In zed cycle, OpenStack projects and oslo lib have dropped
the py3.6 and py3.7 support:

- https://governance.openstack.org/tc/reference/runtimes/zed.html

Change-Id: Iaa3a96e20dc8aeb89d237f267612d5a3397b3dd1

.zuul.yaml

@@ -5,8 +5,6 @@
     check:
       jobs:
         - openstack-tox-pep8
-        - openstack-tox-py36
-        - openstack-tox-py37
         - openstack-tox-py38
         - openstack-tox-py39
         - openstack-tox-py310:
@@ -14,7 +12,5 @@
     gate:
       jobs:
         - openstack-tox-pep8
-        - openstack-tox-py36
-        - openstack-tox-py37
         - openstack-tox-py38
         - openstack-tox-py39

releasenotes/notes/Drop-py3.6-and-py3.7-from-devstack-tools-92af6155b0dc3314.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 3.6 and 3.7 support has been dropped. Last release of devstack-tools
+    to support python 3.6 and 3.7 is devstack-tools 1.2.1. The minimum version
+    of Python now supported by devstack-tools is Python 3.8.

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://opendev.org/openstack/devstack-tools/src/branch/master/README.rst
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -17,8 +17,6 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9