Update python testing as per zed cycle teting runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Removing the py36 centos8 job as well as
updating the python classifier also to reflect the same.

[1] https://governance.openstack.org/tc/reference/runtimes/zed.html

Change-Id: Ie3c38ca062b2bce644d11c48403c9c77ca0de9a1

.zuul.yaml

@@ -6,14 +6,10 @@
     check:
       jobs:
         - openstack-tox-pep8
-        - openstack-tox-py36
-        - openstack-tox-py37
         - openstack-tox-py38
         - openstack-tox-py39
     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-coverage2sql-92af6155b0dc3314.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.6 and 3.7 support has been dropped. The minimum version
+    of Python now supported by coverage2sql is Python 3.8.

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/coverage2sql/latest/
-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