If you use this software, please cite one of the publications that describe it. For most people the 2019 paper is probably the most relevant (the papers from 2009, 2007 did not mention Builder at all, for instance).
Peirce, J. W., Gray, J. R., Simpson, S., MacAskill, M. R., Höchenberger, R., Sogo, H., Kastman, E., Lindeløv, J. (2019). PsychoPy2: experiments in behavior made easy. Behavior Research Methods. 10.3758/s13428-018-01193-y
Peirce, J. W., & MacAskill, M. R. (2018). Building Experiments in PsychoPy. London: Sage.
Peirce J. W. (2009). Generating stimuli for neuroscience using PsychoPy. Frontiers in Neuroinformatics, 2 (10), 1-8. doi:10.3389/neuro.11.010.2008
Peirce, J. W. (2007). PsychoPy - Psychophysics software in Python. Journal of Neuroscience Methods, 162 (1-2):8-13 doi:10.1016/j.jneumeth.2006.11.017
Citing these papers gives the reviewer/reader of your study information about how the system works and it attributes some credit for its original creation. Academic assessment (whether for promotion or even getting appointed to a job in the first place) prioritises publications over making useful tools for others. Citations provide a way for the developers to justify their continued involvement in the development of the package.
PsychoPy is licensed under a GPL3 license which means, essentially, that:
you can use it (and adapt it) for free in your work, and you can even release those versions
but you must include the original PsychoPy license
AND you must also make your release open source using the same license
What that means is you’re free to use PsychoPy’s goodwill in being open source, you are required to pass on that goodwill!