replace whitelist_externals by allowlist_externals

replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Depends-on: https://review.opendev.org/c/openstack/designate/+/796597
Change-Id: Id28a67abc74c3e14d4cba8d3278c89a1fe029252

tox.ini

@@ -35,7 +35,7 @@ deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
-whitelist_externals =
+allowlist_externals =
   sh
   find
   rm
@@ -50,7 +50,7 @@ commands =
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf
@@ -97,7 +97,7 @@ commands = oslopolicy-sample-generator --config-file etc/designate/designate-pol
 
 [testenv:bashate]
 deps = bashate
-whitelist_externals = bash
+allowlist_externals = bash
 commands = bash -c "find {toxinidir}/devstack         \
          -not \( -type d -name .?\* -prune \)         \
          -not \( -type d -name doc -prune \)          \