Add Oman VAT number (VATIN) validation

[DMZ22]

Partially addresses #408 by adding the Oman VATIN — the VAT/Excise form of the Oman TIN, which is the one with a defined structure.

Format

• Name: VATIN (VAT identification number), issued by the Oman Tax Authority.
• Structure: the letters OM followed by 10 digits — 12 characters in total.
• No check digit. Numbers are verified online via the tax authority portal, so this module validates length and formatting only (in the spirit of e.g. stdnum/gh/tin.py, which is also structural).

Sources / validation service

• https://tms.taxoman.gov.om/portal/web/taxportal/vatin-validation (official VATIN validation)
• OECD CRS TIN guidance for Oman describes the 12-character VAT/Excise TIN.

Tests

• Module docstring covers valid input, whitespace/lowercase normalisation, missing prefix, wrong length and non-digit tail.
• tests/test_om_vat.doctest bulk-validates real numbers taken from published Omani VAT certificates (e.g. OM1100006083, OM1100018164, OM1100005523). Every line of the module is exercised.

Note

The #408 issue also mentions a 7-digit "tax card number" for entities. I left that out: it's an unstructured all-digit sequence with no fixed length constraint or check digit, so validating it adds little (per CONTRIBUTING's guidance on numbers whose only spec is "N digits"). Happy to add it, or to fold both into an om/tin.py, if you'd prefer.

[arthurdejong]

Hi @DMZ22,

Thanks for contributing this. The changes were merged as 9f1bc4a. The only significant change I made was to ensure that numbers that end with X are also supported (they were found "in the wild" and confirmed to be valid via the validation service).

That triggered me to reverse engineer the check digit algorithm in 23be163. I did not manage to find weights for the first 4 digits (after OM) as there was insufficient variation there but this algorithm works for all the valid numbers I managed to find.