Replace deprecated whitelist_externals

The whitelist_externals option has been deprecated since tox 3.18.0[1].

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

Change-Id: I6a0b056bd944d93fb6afd9a1671ad1fa8010e251

tox.ini

@@ -15,7 +15,7 @@ deps =
 setenv =
     VIRTUAL_ENV={envdir}
   PYTHONDONTWRITEBYTECODE=1
-whitelist_externals = sh
+allowlist_externals = sh
                       find
                       rm
 commands =
@@ -43,14 +43,14 @@ commands = rm -rf doc/build
 
 [testenv:releasenotes]
 deps = {[testenv:docs]deps}
-whitelist_externals = rm
+allowlist_externals = rm
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:bashate]
 deps = bashate
-whitelist_externals = bash
+allowlist_externals = bash
 commands = bash -c "find {toxinidir}/devstack         \
          -not \( -type d -name .?\* -prune \)         \
          -not \( -type d -name doc -prune \)          \