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: I83717c5b2661df0d4288699903c38814cefb43d9

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = py38,pep8
-minversion = 2.3.2
+minversion = 3.18.0
 skipsdist = True
 ignore_basepython_conflict = True
 
@@ -23,7 +23,7 @@ commands = sh -c "find . -type d -name '.?*' -prune -o \
            \( -type d -name '__pycache__' -o -type f -name '*.py[co]' \) \
            -print0 | xargs -0 rm -rf"
            stestr run {posargs}
-whitelist_externals = sh
+allowlist_externals = sh
 
 [testenv:pep8]
 commands =
@@ -59,7 +59,7 @@ commands = sphinx-build -W -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 -b latex doc/source doc/build/pdf