Devops: Update deprecated whitelist_externals option in tox config (#…

…6380)

The option `whitelist_externals` is deprecated in tox with version 3.18
(Release Jul 23, 2020). The option `allowlist_externals` is used since
3.18. Pinning 3.18 as minimal version for tox.

pyproject.toml

@@ -412,6 +412,7 @@ select = [
 [tool.tox]
 legacy_tox_ini = """
 [tox]
+minversion = 3.18.0
 envlist = py39
 
 [testenv]
@@ -444,7 +445,7 @@ passenv = RUN_APIDOC
 setenv =
     update: RUN_APIDOC = False
 changedir = docs
-whitelist_externals = make
+allowlist_externals = make
 commands =
     clean: make clean
     make debug