Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 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: I1cb8eadf6b649329e8faf5e63a32b2b017b01ebf

Author: wu.shiming

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.18.0
 envlist = py36,py38,pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -38,7 +38,7 @@ commands = sphinx-build -W --keep-going -b html doc/source doc/build/html
 [testenv:pdf-docs]
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W --keep-going -b latex doc/source doc/build/pdf