doc: remove `data_nbytes` from Python Producer client documentation
... by making data_nbytes a C function instead of a Python function.
See
https://notes-on-cython.readthedocs.io/en/latest/function_declarations.html
for the difference between def and cdef in Cython code.
From the current (05/06/2024) documentation https://asapo.pages.desy.de/asapo/python/:
Edited by Marc-Olivier Andrez