Fix tox.ini for tox4

This patch also replaces the outdated py36 job with the py38 one.

Change-Id: Id330d8cdbcf70b0403e8283228ce9dec10403012

.zuul.yaml

@@ -3,13 +3,13 @@
       - openstack-specs-jobs
     check:
       jobs:
-        - openstack-tox-py36:
+        - 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-py36:
+        - openstack-tox-py38:
             irrelevant-files:
               - ^releasenotes/.*$

tox.ini

@@ -1,7 +1,6 @@
 [tox]
-minversion = 2.0
-envlist = docs,py36
-skipsdist = True
+minversion = 3.18.0
+envlist = docs,py38
 
 [testenv]
 basepython = python3
@@ -22,9 +21,10 @@ commands = stestr run --slowest {posargs}
 commands = {posargs}
 
 [testenv:docs]
+allowlist_externals = rm
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
 commands =
-  /bin/rm -fr doc/build/
+  rm -fr doc/build/
   sphinx-build -W -b html doc/source doc/build/html