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: Ibb77fa2afad3f09e95f0dba243d3a096daedd787

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.2.0
+minversion = 3.18.0
 envlist = py38,pep8
 skipdist = True
 # Automatic envs (pyXX) will only use the python version appropriate to that
@@ -18,7 +18,7 @@ deps =
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 commands = stestr run {posargs}
-whitelist_externals = stestr
+allowlist_externals = stestr
 
 [testenv:fast8]
 # Use same environment directory as pep8 env to save space and install time
@@ -71,7 +71,7 @@ commands =
     pythom -m pip install -q -e "git+file://{toxinidir}/../openstacksdk#egg=openstacksdk"
     python -m pip freeze
     stestr run {posargs}
-whitelist_externals = stestr
+allowlist_externals = stestr
 
 [testenv:functional]
 setenv = OS_TEST_PATH=./openstackclient/tests/functional