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

Change-Id: I004d9563d006a1cce1e2fe7fbcd11fbf41e023fe
Signed-off-by: Takashi Natsume <takanattie@gmail.com>

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py39,functional,pep8
 skipsdist = true
 # Automatic envs (pyXX) will use the python version appropriate to that
@@ -10,7 +10,7 @@ ignore_basepython_conflict = true
 [testenv]
 basepython = python3
 usedevelop = true
-whitelist_externals =
+allowlist_externals =
   rm
 setenv =
   PYTHONDONTWRITEBYTECODE=1
@@ -94,7 +94,7 @@ commands =
 description =
   Build PDF documentation.
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf