Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 python 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: Icea0fbbd3e389800d7fcfdd6ecd11f3b05b2a892

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = bashate
-minversion = 2.0
+minversion = 3.18.0
 skipsdist = True
 
 [testenv]
@@ -19,7 +19,7 @@ commands =
   bash -c "ls *.sh prep-* stop-* upgrade-* from-*/upgrade-* functions | xargs bashate -v {posargs}"
 
 [testenv:docs]
-whitelist_externals = bash
+allowlist_externals = bash
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
@@ -32,7 +32,7 @@ commands =
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
    make
 commands =
    sphinx-build -W -b latex doc/source doc/build/pdf