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: I86c077215e3256a050d22b00e89d240ef7dbcbff

tox.ini

@@ -1,11 +1,11 @@
 [tox]
-minversion = 3.1
+minversion = 3.18.0
 envlist = py3,pep8
 ignore_basepython_conflict = true
 
 [testenv]
 basepython = python3
-whitelist_externals =
+allowlist_externals =
   env
 passenv =
   OS_TEST_DBAPI_ADMIN_CONNECTION
@@ -41,7 +41,7 @@ commands =
   coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-whitelist_externals =
+allowlist_externals =
   rm
 deps =
   {[testenv]deps}
@@ -52,7 +52,7 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:releasenotes]
-whitelist_externals =
+allowlist_externals =
   rm
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}