Introduce validator for GB UTR

[MuhistheHolvi]

Introduce validator for GB UTR.

[arthurdejong]

HI @MuhistheHolvi the compact() function is missing (which is one of the causes of the test failures) and there are a few flake8 issues.

[MuhistheHolvi]

I need to learn using doctest locally. This constant pushing is not sustainable after every couple lines of change.

[MuhistheHolvi]

@arthurdejong Thank you for the pointer. I think I will close this PR when done and open new one with one commit. I think I am approaching one commit per line of code :) Unless you do a squat merge, in that case please go ahead and merge this branch.

[arthurdejong]

HI @MuhistheHolvi,

Sorry for not responding sooner.

You should be able to run the tests locally with tox (e.g. use tox -e py36 to run it with a Python 3.6 interpreter, and use tox -e flake8 to run the style tests). There are still a few issues:

• it only accepts numbers with 9 digits in length while it the documentation (and other sources) claim the number is 10 digits
• it does not strip commonly a start or end 'k' from the number as described on https://design.tax.service.gov.uk/hmrc-design-patterns/unique-taxpayer-reference/
• while an algorithm for the UTR is not well-documented, an explicitly denied FOI request did publish the algorithm via an online checker: https://www.whatdotheyknow.com/request/unique_tax_reference_number (algorithm also found in a few other places)

A function to calculate a check digit based on the above would be (note that the check digit is the first digit, not the last one which is more common):

def calc_check_digit(number):
    """Calculate the check digit for the number. The passed number should not
    have the check digit (the first one) included."""
    weights = (6, 7, 8, 9, 10, 5, 4, 3, 2)
    return '21987654321'[sum(int(n) * w for n, w in zip(number, weights)) % 11]

The above implementation also seems to match the one used on https://easydigitalfiling.com/kb/what-is-a-utr-number

These generated numbers are treated as correct on both implementations:

1234567895
1478641088
1955839661
2172858530
2234567890
4748949890
5179265754
5816619590
8258106771
9624088948