Hi @dohrann - sorry for the confusion I introduced by mentioning Python.
Workflows Custom JavaScript Libraries is available on 3.75 as a beta (community post about it is here) however as the (latest release) docs mention, you will need the code-executor container running for custom JavaScript libraries as well. The fact that you do not see Python as an option in your workflows UI means that it appears your self-hosted deployment does not have a code-executor container running.
The Workflows Custom JavaScript is already out of beta for cloud-hosted customers and will be out of beta for self-hosted deployments in the next Retool stable release.
If you get code-executor running and you'd like us to enable the beta for you, please DM me the email associated with your self-hosted license.