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: Ia986290812e8f577cb89477d20a169f8aafd91df

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist = py36,py37,py38,py39,pep8
 ignore_basepython_conflict = True
-minversion = 3.1.0
+minversion = 3.18.0
 skipsdist = True
 
 [testenv]
@@ -55,7 +55,7 @@ commands =
   coverage report
 
 [testenv:docs]
-whitelist_externals =
+allowlist_externals =
   rm
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
@@ -66,7 +66,7 @@ commands =
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf
@@ -76,7 +76,7 @@ commands =
 # This environment is called from CI scripts to test and publish
 # the API Ref to docs.openstack.org.
 deps = -r{toxinidir}/doc/requirements.txt
-whitelist_externals = rm
+allowlist_externals = rm
 commands =
   rm -rf api-ref/build
   sphinx-build -W --keep-going -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
@@ -136,7 +136,7 @@ paths = ./heat/hacking
 commands = oslo_debug_helper {posargs}
 
 [testenv:releasenotes]
-whitelist_externals =
+allowlist_externals =
   rm
 deps = -r{toxinidir}/doc/requirements.txt
 commands =