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: Ia637e459b9401ab3f07fdf6e88868080ce59e245

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 distribute = False
 envlist = py36,py38,pep8
-minversion = 3.1.0
+minversion = 3.18.0
 skipsdist = True
 skip_missing_interpreters = true
 # this allows tox to infer the base python from the environment name
@@ -27,7 +27,7 @@ deps =
 commands = find . -type f -name "*.pyc" -delete
            stestr run {posargs}
            stestr slowest
-whitelist_externals = find
+allowlist_externals = find
 
 [testenv:pep8]
 commands =
@@ -40,7 +40,7 @@ deps =
   -r{toxinidir}/requirements.txt
        pylint==1.9.1
 commands = bash tools/lintstack.sh
-whitelist_externals = bash
+allowlist_externals = bash
 
 [testenv:venv]
 commands = {posargs}
@@ -69,7 +69,7 @@ commands =
   {[testenv:docs]commands}
   sphinx-build -W -b latex doc/source doc/build/pdf
   make -C doc/build/pdf
-whitelist_externals =
+allowlist_externals =
   make
   cp