Switch to allowlist_externals in tox configuration

The old whitelist_externals setting was deprecated for a long time and
got removed in the meantime [1].

[1]: tox-dev/tox#2600

tox.ini

@@ -25,7 +25,7 @@ commands =
     python setup.py sdist bdist_wheel
 
 [testenv:update-requirements]
-whitelist_externals =
+allowlist_externals =
     bash
 deps =
     pipenv
@@ -35,8 +35,8 @@ commands =
     bash -c 'pipenv requirements --dev > "{toxinidir}/test-requirements.txt"'
 
 [testenv:pipenv]
-whitelist_externals =
-    {[testenv:update-requirements]whitelist_externals}
+allowlist_externals =
+    {[testenv:update-requirements]allowlist_externals}
 deps =
     {[testenv:update-requirements]deps}
 commands =