Bump tox minversion to 3.18.0

Since tox 3.18.0, the whitelist_externals option has been deprecated in
favor of the new allow_list_externals option[1].

This change bumps the minversion of tox so that we can replace
the deprecated option.

[1] https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: I1943ddbe8d63c577b5c8084a158af5bae4d1f9fc

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1
+minversion = 3.18.0
 envlist = pep8,py36,py39,releasenotes,npm,py3-dj32
 skipsdist = True
 # Automatic envs (pyXX) will only use the python version appropriate to that
@@ -21,7 +21,7 @@ setenv =
   # It should be dropped when we drop Django 2.2 support.
   dj32: PYTHONWARNINGS = once,ignore::PendingDeprecationWarning,ignore::DeprecationWarning
 
-whitelist_externals =
+allowlist_externals =
   bash
   find
 deps =
@@ -64,7 +64,7 @@ commands =
 
 [testenv:cover]
 envdir = {toxworkdir}/venv
-whitelist_externals =
+allowlist_externals =
   bash
 commands =
   coverage erase
@@ -148,7 +148,7 @@ commands =
 [testenv:pdf-docs]
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -j auto -W -b latex doc/source doc/build/pdf