Drop python 3.6 testing

Python 3.6 is sufficiently old at this point that we don't need to keep
testing on it. This will reduce the number of things we need to tend and
care for with this application and service.

Change-Id: Iadea5270f54631ef72a68a37073fd8d337bc4c5c

.zuul.yaml

@@ -59,8 +59,6 @@
     check:
       jobs:
         - tox-pep8
-        - tox-py36:
-            nodeset: ubuntu-bionic
         - tox-py38:
             nodeset: ubuntu-focal
         - tox-py310
@@ -69,8 +67,6 @@
     gate:
       jobs:
         - tox-pep8
-        - tox-py36:
-            nodeset: ubuntu-bionic
         - tox-py38:
             nodeset: ubuntu-focal
         - tox-py310

setup.cfg

@@ -11,8 +11,6 @@ classifier =
     Intended Audience :: Information Technology
     License :: OSI Approved :: BSD License
     Programming Language :: Python
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = pep8, py36, py38, py310
+envlist = pep8, py38, py310
 skipsdist = True
 
 [testenv]