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: Ie0d1d3964043e6dd518f3b7020f58f4f3b2de76c

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py{38,36},pep8,pip-check-reqs
 skipsdist = True
 ignore_basepython_conflict = True
@@ -8,7 +8,7 @@ ignore_basepython_conflict = True
 basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
-whitelist_externals = find
+allowlist_externals = find
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
@@ -46,7 +46,7 @@ commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
    make
 commands =
    sphinx-build -W -b latex doc/source doc/build/pdf