Change minversion of tox to 3.18.0

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

Signed-off-by: Takashi Natsume <takanattie@gmail.com>
Change-Id: Icf591aabde39489f539f2ca09ee59ba7c58e98a7

tox.ini

@@ -1,13 +1,13 @@
 [tox]
 envlist = py36,py38,pep8
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
 usedevelop = True
-whitelist_externals = rm
+allowlist_externals = rm
 deps =
        -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
@@ -48,7 +48,7 @@ commands =
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals = make
+allowlist_externals = make
 commands =
     sphinx-build --keep-going -b latex doc/source doc/build/pdf
     make -C doc/build/pdf