WIP: fixup: add compatibility with missing __code__ attribute
In order to apply https://lab.nexedi.com/nexedi/erp5/commit/4115e4658f50a48d3cd4b10e0ae033a3aaa3e273 it is needed to upgrade an existing side which only contains func_code
In order to apply https://lab.nexedi.com/nexedi/erp5/commit/4115e4658f50a48d3cd4b10e0ae033a3aaa3e273 it is needed to upgrade an existing side which only contains func_code