erp5_wendelin: do not load pickles when converting wendelin text format to numpy

Base_wendelinTextToNumpy just takes a string and pass it to numpy.load,
but numpy.load will load pickles (problem from https://github.com/numpy/numpy/issues/12759 ).
Therefore we disallow loading pickles here until this becomes the
default in newer numpy versions.
Co-authored-by: Jérome Perrin <jerome@nexedi.com>