ci: Replace whitelist_externals with allowlist_externals and removed whitelist_externals.

[simkimsia]

tox replaced whitelist_externals with allowlist_externals and removed whitelist_externals.

This PR fixes the tox.ini so that it uses the new option.

See also: tox.wiki/en/3.18.1/changelog.html#v3-18-0-2020-07-23
and python-poetry/poetry#7341