Change minversion of tox to 3.18.0

The patch bumps min version of tox to 3.18.0 in order to
replace whitelist_externals with allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Signed-off-by: Takashi Natsume <takanattie@gmail.com>
Change-Id: I663c05734cfc86a3c2bbc068c3990bab037bbe22

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.2
+minversion = 3.18.0
 envlist = py37,py39,pep8
 ignore_basepython_conflict = True
 
@@ -39,7 +39,7 @@ commands = sphinx-build -W -b html doc/source doc/build/html
 [testenv:pdf-docs]
 basepython = python3
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf