ci: no module found for coverage when switch to allowlist_externals in tox.ini

Reproduced in https://stackoverflow.com/questions/75910092/how-to-fix-tox-ini-when-it-keeps-complaining-about-no-module-named-coverage

See the relevant PR https://github.com/torchbox/django-pattern-library/pull/213

and related to https://github.com/torchbox/django-pattern-library/pull/212 after allowlist_externals replaced whitelist_externals