Remove the `bandit` version constraint when not needed anymore in the flake8-requirements file

[smarie]

See https://github.com/tylerwince/flake8-bandit/issues/21

Currently our fix is to add bandit<1.7.3 in the flake8 requirements file

https://github.com/smarie/python-makefun/blob/main/ci_tools/flake8-requirements.txt