[FIX] core: PEP 420 compatibility

[xmo-odoo]

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.

[robodoo]

[rven]

What is the status of this PR?

Odoo doesn't start anymore when the package is installed.

initialize_sys_path
    odoo.upgrade.__path__._path_finder = lambda *a: None  # prevent path invalidation
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'list' object has no attribute '_path_finder'

[aj-fuentes]

Rebased to let the CI test it.

[aj-fuentes]

upgradeci retry with base account in all versions

[aj-fuentes]

@KangOl I confirm it works while upgrading a DB from 7.0 up to 19.0 both locally and in the platform. If the CI is all good this looks good to go.