I created an initial package to run prosemirror transformations in Python code  with a simple test of applying a step to a doc that had been loaded previously and then turning the doc back into JSON .
Translating the relevant parts of
prosemirror-transform to Python code using Js2Py  seems to be working.
I have put it under the AGPL license so far, as I’m not really interested in maintaining it if other closed-source projects are going to use it without giving back, but I would be open to change the license to something else if someone else is willing to take on maintenance, writing tests, expanding on it, etc. .