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

Change-Id: I00b6af1560c84d0b99b6044078c1d65d22b206b0

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.18.0
 envlist = py3,pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -7,7 +7,7 @@ ignore_basepython_conflict = True
 [testenv]
 basepython = python3
 usedevelop = True
-whitelist_externals = find
+allowlist_externals = find
                       rm
 install_command = pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 setenv =
@@ -55,7 +55,7 @@ commands =
 
 [testenv:api-ref]
 deps = -r{toxinidir}/doc/requirements.txt
-whitelist_externals = bash
+allowlist_externals = bash
 commands =
   bash -c 'rm -rf api-ref/build'
   sphinx-build -W --keep-going -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
@@ -78,7 +78,7 @@ commands = python setup.py bdist_wheel
 [testenv:pdf-docs]
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   rm
   make
 commands =