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: I434a6330defbc5cc65788f4d64b7f2d4d2b37db7

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.9.0
+minversion = 3.18.0
 skipsdist = True
 envlist = py3,functional,pep8
 ignore_basepython_conflict=true
@@ -40,7 +40,7 @@ deps=
     flake8-import-order>=0.17.1 # LGPLv3
     pycodestyle>=2.0.0,<2.7.0 # MIT
     doc8>=0.8.1 # Apache-2.0
-whitelist_externals = bash
+allowlist_externals = bash
 commands =
   flake8 {posargs:ironic_python_agent examples}
   # Run bashate during pep8 runs to ensure violations are caught by
@@ -78,7 +78,7 @@ commands =
   sphinx-build -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-whitelist_externals = make
+allowlist_externals = make
 setenv = PYTHONHASHSEED=0
 sitepackages = False
 deps = {[testenv:docs]deps}
@@ -120,7 +120,7 @@ commands =
 [testenv:genconfig]
 sitepackages = False
 envdir = {toxworkdir}/venv
-whitelist_externals = mkdir
+allowlist_externals = mkdir
 commands =
   mkdir -p etc/ironic_python_agent
   oslo-config-generator --config-file=tools/config/ipa-config-generator.conf