Update min version of tox to use allowlist

The min version of tox is updated to 3.18.0 to replace whitelist_externals
with allowist_externals option [1]

[1] https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: Icf821d49d2543be2a37dffc686ae7ab4163b04cc

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.9.0
+minversion = 3.18.0
 skipsdist = True
 envlist = py3,pep8
 ignore_basepython_conflict=true
@@ -39,7 +39,7 @@ deps=
     flake8-import-order>=0.17.1 # LGPLv3
     Pygments>=2.2.0 # BSD
     bashate>=0.5.1 # Apache-2.0
-whitelist_externals = bash
+allowlist_externals = bash
 commands =
   bash tools/flake8wrap.sh {posargs}
   # Run bashate during pep8 runs to ensure violations are caught by
@@ -85,7 +85,7 @@ deps =
 commands = sphinx-build -b html -W doc/source doc/build/html
 
 [testenv:pdf-docs]
-whitelist_externals = make
+allowlist_externals = make
 deps = {[testenv:docs]deps}
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf
@@ -97,7 +97,7 @@ usedevelop = False
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
-whitelist_externals = bash
+allowlist_externals = bash
 commands =
   bash -c 'rm -rf api-ref/build'
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html