Pylint alerts corrections as part of intervention experiment

[evidencebp]

I'd like to conduct a software engineering experiment regarding the benefit of Pylint alerts removal.
The experiment is described here.
In the experiments, Pylint is used with some specific alerts, files are selected for intervention and control.
After the interventions are done, one can wait and examine the results.

I'm asking for your approval for conducting an intervention in your repository.

See examples of interventions in stanford-oval/storm, gabfl/vault, and coreruleset/coreruleset.

In this project the intervention is very simple, fixing a single alert.
Therefore I created the PR already.
The PR simplifies a if statement.

If you are OK with the PR I'll be very happy if you will merge it so I'll be able to observe the outcome of the fix.

[evidencebp]

@alanhamlett @jarek-t I noticed that you worked on the intervention file, filter.py

Are you OK with the PR?