[FIX] port base_gs1_barcode to v7

Make base_gs1_barcode installable.
Relicence as AGPL-3.
Match coding conventions more closely
Use a distinct exception. This exception is placed in the package's scope to make it easier to use from other modules. Now use "from base_gs1_barcode import InvalidGS1Barcode"
Why do you need this here?