Add support for XI VAT numbers in vatin

[alexis-via]

XI VAT numbers now works well via stdnum.gb.vat, but not via stdnum.vatin
This PR fixes it.
I'm not familiar with the code, so check carefully.
More info on #250

[arthurdejong]

Thanks, I've merged it as 53f13b4 with a small modification and the addition of a test.

[alexis-via]

Thank you very much! Would it be possible to have a new release of python-stdnum ? It would be a kind of "Brexit" release :) It's important for the software that rely on stdnum to validate VAT numbers and that must now accept VAT numbers for Northern Ireland.

[alexis-via]

@arthurdejong What do you think of my suggestion for a new "Brexit compliant" release of stdnum (1.16) ? If I can help make that happen, please tell me.
It would be very useful for several opensource software that rely on stdnum to validate VAT numbers and now need to enter XI VAT numbers.

[arthurdejong]

Hi @alexis-via,

I would also like to incorporate the changes in #248 but I'm still waiting for a few missing bits of information. After that I will make a release. If there is no response by this weekend I'll also make a new release to not delay the brexit-changes rollout any longer.

Other than that there is not much to do. Making the release is just a matter of following by quick checklist, making release notes, signing stuff, uploading, etc.

[alexis-via]

OK, thanks !

[arthurdejong]

FYI: Release 1.16 was just made and includes this fix.

[alexis-via]

Thank you so much for this new release! It works great.