Revert "erp5: py3: `func_{code,defaults}` was replaced in Python3 by `__{code,defaults}__`."

This reverts commit 4fa33dfc.

This patch is not complete, backporting full support of
`__{code,defaults}__` on Zope2 would require patching more, so we
decided to keep using func_code with ERP5 on Zope2 and to use
__code__ only with ERP5 on Zope4