Bump tox minversion to 3.18.0

Since tox 3.18.0, the whitelist_externals option has been deprecated in
favor of the new allow_list_externals option[1].

This change bumps the minversion of tox so that we can replace
the deprecated option.

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

Change-Id: I5aa8d079fc8e132cd90293b889ed53cb7d46da19

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.2.0
+minversion = 3.18.0
 skipsdist = True
 envlist = py38,pep8,releasenotes
 ignore_basepython_conflict = True
@@ -15,7 +15,7 @@ deps =
        -r{toxinidir}/test-requirements.txt
 commands = find . -type f -name "*.pyc" -delete
            stestr run --slowest {posargs}
-whitelist_externals = find
+allowlist_externals = find
 basepython = python3
 
 [testenv:pep8]
@@ -68,7 +68,7 @@ deps = -r{toxinidir}/doc/requirements.txt
 [testenv:pdf-docs]
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
   rm
 commands =