fix bandit checks

[ana]

Bandit is a tool designed to find common security issues in Python code https://github.com/PyCQA/bandit.
It can be installed with pip install bandit and it can run in avocado with bandit -r -lll .

At this moment, the metrics are:

...
Run metrics:
        Total issues (by severity):
                Undefined: 0.0
                Low: 85.0
                Medium: 97.0
                High: 4.0
        Total issues (by confidence):
                Undefined: 0.0
                Low: 57.0
                Medium: 16.0
                High: 113.0

It would be nice to see if the medium and high issues can be solved.
Note that there is a PR already in progress with some fixes at #5256