tests fail to run with pippy is not allowed

[jaraco]

Running tox at master:

 csv2ofx main @ tox
py: install_deps> helpers/pippy -r /Users/jaraco/code/reubano/csv2ofx/dev-requirements.txt -r /Users/jaraco/code/reubano/csv2ofx/requirements.txt
py: failed with /Users/jaraco/code/reubano/csv2ofx/helpers/pippy (resolves to /Users/jaraco/code/reubano/csv2ofx/helpers/pippy) is not allowed, use allowlist_externals to allow it
  py: FAIL code 1 (0.15 seconds)
  evaluation failed :( (0.19 seconds)

[jaraco]

This is a dupe of #102.