Update min version of tox

This patch bumps the minimum version of tox to 3.18.0 [1].
Additionally fix the argument such as "whitelist_externals"
which is deprecated, it replaces whitelist_externals by
allowlist_externals option [1].

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

Change-Id: I80b180a433636d1ac70546535f1d3762f828cb13

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.18.0
 envlist = pep8,docs
 skipsdist = True
 
@@ -16,7 +16,7 @@ commands = {posargs}
 
 [testenv:docs]
 deps = -r{toxinidir}/doc/requirements.txt
-whitelist_externals = rm
+allowlist_externals = rm
 commands =
   rm -fr doc/build
   sphinx-build -W -b html doc/source doc/build/html