tox fails with pippy is not allowed

[jaraco]

 meza main @ tox
py: install_deps> helpers/pippy -r /Users/jaraco/code/reubano/meza/dev-requirements.txt -r /Users/jaraco/code/reubano/meza/requirements.txt
py: failed with /Users/jaraco/code/reubano/meza/helpers/pippy (resolves to /Users/jaraco/code/reubano/meza/helpers/pippy) is not allowed, use allowlist_externals to allow it
  py: FAIL code 1 (0.15 seconds)
  evaluation failed :( (0.20 seconds)

Same as reubano/csv2ofx#102.