Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 python 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: I954af0724274ec6f606a7a46d39a7ef9f528809f

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.3.1
+minversion = 3.18.0
 envlist = pep8,py{27,36,38}
 skipsdist = True
 skip_missing_interpreters = True
@@ -13,7 +13,7 @@ setenv =
   VIRTUAL_ENV = {envdir}
 usedevelop = True
 install_command = pip install -U {opts} {packages}
-whitelist_externals = *
+allowlist_externals = *
 deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt