[Python] Experiment with newer Python multipart parsers #51759

At the time when we originally worked on #40598 and #40599, we faced the problem that Python's standard email.parser was (so far as we knew at the time) the only established implementation of multipart parsing for Python, and it performed very poorly. Now there are other established implementations, including:

• https://github.com/Kludex/python-multipart (this is the one FastAPI / Starlette use)
• https://github.com/defnull/multipart (this one seems to be more performant)

Implement examples using these.