[FIX] core: PEP 420 compatibility

odoo/odoo#195664 removed explicit namespace support, as a result the
`__init__` files break the ability to hook upgrade in via
`PYTHONPATH`, as under PEP420 no namespace entry may have a`__init__`
files within the shared namespace path.

closes #235

Signed-off-by: Christophe Simonis (chs) <chs@odoo.com>

Author: xmo-odoo