Update to use openstack-python3-jobs template

Using openstack-python3-jobs will allow this to
seamlessly move forward with the python version,
which currently should be py39 and py312.

Removed the py38 override as it will not be used
any more, and for reference, the template does
contain a similar irrelevant-files line so that
is not lost.

TrivialFix

Change-Id: Ieffe3581dfa444ae5d014d4b499c3ee7694e7e57

.zuul.yaml

@@ -1,15 +1,4 @@
 - project:
     templates:
       - openstack-specs-jobs
-    check:
-      jobs:
-        - openstack-tox-py38:
-            # Override default list of files since the default
-            # ignores .rst files but we need to test .rst files here.
-            irrelevant-files:
-              - ^releasenotes/.*$
-    gate:
-      jobs:
-        - openstack-tox-py38:
-            irrelevant-files:
-              - ^releasenotes/.*$
+      - openstack-python3-jobs

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.18.0
-envlist = docs,py3
+envlist = docs,py3,pep8
 ignore_basepython_conflict = True
 
 [testenv]