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: Id8bdda703afc39d352e3a53877318dc30d91a5f7

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 1.6
+minversion = 3.18.0
 skipsdist = True
 envlist = bashate
 
@@ -13,7 +13,7 @@ basepython = python3
 # modified bashate tree
 deps =
    {env:BASHATE_INSTALL_PATH:bashate==2.0.0}
-whitelist_externals = bash
+allowlist_externals = bash
 commands = bash -c "find {toxinidir}             \
          -not \( -type d -name .?\* -prune \)    \
          -not \( -type d -name doc -prune \)     \
@@ -37,15 +37,15 @@ commands = bash -c "find {toxinidir}             \
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
-whitelist_externals = bash
+allowlist_externals = bash
 setenv =
   TOP_DIR={toxinidir}
 commands =
   sphinx-build -W -b html -d doc/build/doctrees doc/source doc/build/html
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
    make
 commands =
    sphinx-build -W -b latex doc/source doc/build/pdf