Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 in order to
replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Also removes skipdist=True as in
Ib0fca00c92ef56fce57eb6355cb6be36e478a744.

Also inlcludes:

Fix issues with tox 4.2.4

tox now hard fails if there is mismatch in spec
attributes[1]. This works around the issue
temporarily. We probably have to drop py38 jobs
and specify basepython for py39 target in the
future.

[1] https://github.com/tox-dev/tox/pull/2824/

Conflicts:
	tox.ini

Backport note:
This is now required to adapt to the new tox 4.0 release, which no
longer supports the old deprecated options.

Closes-Bug: #2002035
Change-Id: I5792c86745b875d074f670d92fe78a84a907bd74
(cherry picked from commit 3cb006d532
 and 7fd4519b22)
(cherry picked from commit 0c0def6f81)

tox.ini

@@ -1,7 +1,7 @@
 [tox]
-minversion = 2.0
-skipsdist = True
+minversion = 3.18.0
 envlist = pep8,py,tht
+ignore_base_python_conflict = True
 
 [testenv]
 basepython = python3
@@ -21,7 +21,7 @@ setenv =
 commands = {posargs}
 
 [testenv:pep8]
-whitelist_externals =
+allowlist_externals =
     bash
 commands =
     python ./tools/process-templates.py
@@ -68,7 +68,7 @@ commands = python ./tools/process-templates.py -c
 [testenv:releasenotes]
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/wallaby}
        -r{toxinidir}/doc/requirements.txt
-whitelist_externals =
+allowlist_externals =
     bash
 commands = bash -c tools/releasenotes_tox.sh
 
@@ -89,7 +89,7 @@ commands =
            python ./tripleo_heat_templates/environment_generator.py sample-env-generator/
 
 [testenv:genroledata]
-whitelist_externals =
+allowlist_externals =
     bash
 commands =
     bash -c tools/roles-data-generate-samples.sh
@@ -113,7 +113,7 @@ passenv =
 deps =
   -r{toxinidir}/test-ansible-requirements.txt
   -r{toxinidir}/test-requirements.txt
-whitelist_externals =
+allowlist_externals =
     bash
 commands_pre =
     ansible-galaxy collection install ansible.posix containers.podman