Run bandit on specutils and integrate it into the CI

[Bandit](https://pypi.org/project/bandit/) is a now I think fairly standard tool for auditing Python packages for known security issues. We should try running that on specutils to make sure there aren't issues, and if not set it up to run on new PRs as part of the CI. (There's a pretty much ready-to-go github action for this it looks like, at least based on what I see @pllim implemented in https://github.com/spacetelescope/synphot_refactor).

Start by running Bandit against the specutils package and reviewing any reported security issues. Read the ready-to-go GitHub Action example referenced from synphot_refactor, then configure the repository's CI to run it on new pull requests. Done means the existing code has been checked and Bandit runs successfully for future PRs.

Written by the indexing model from the issue text.