Flake8 is erroring out in CI runs

[digitaldogsbody]

Flake8 has started erroring out in CI runs with the following error:

ValueError: Error code '#' supplied to 'ignore' option does not match '^[A-Z]{1,3}[0-9]{0,3}$'
Error: ERROR: InvocationError for command /home/runner/work/iiif-prezi3/iiif-prezi3/.tox/linting/bin/flake8 (exited with code 1)

I could not reproduce this locally, and no files that flake8 touches have changed, but searching the error message turned up this thread:
https://stackoverflow.com/questions/74558565/flake8-error-code-supplied-to-ignore-option-does-not-match-a-z1-30 which suggests that the recent upgrade to flake8 6.0.0 is the cause.

Hopefully we can fix this in the tox config, and the implementation of #98 should prevent this sort of issue from happening again.