Update min version of tox to use allowlist

The min version of tox is updated to 3.18.0 to replace whitelist_externals
with allowist_externals option [1]

[1] https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: I42091bde2be25a52399fe66c2ca4600274dd0350

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.9.0
+minversion = 3.18.0
 envlist = pep8
 skipsdist = True
 ignore_basepython_conflict = true
@@ -33,7 +33,7 @@ commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 usedevelop = False
-whitelist_externals = make
+allowlist_externals = make
 commands = sphinx-build -b latex doc/source doc/build/pdf
            make -C doc/build/pdf