Use rsplit instead of rstrip to get old_wp_names

[dracic]

Resolves #20

[wonder-sk]

Actually using rsplit can also produce incorrect results:

>>> "hello.gpkg.world.gpkg".rsplit('.gpkg')
['hello', '.world', '']

From python 3.9 there is removesuffix() method, but that's still quite new version - I will fix it with filename[:-5] for the time being...

[wonder-sk]

Fixed in df7c4a2