'Returns 9 character Polish National Business Registry Number,
Polish: Rejestr Gospodarki Narodowej - REGON.
https://pl.wikipedia.org/wiki/REGON'

@classmethod
def regon(cls):

    voivodeship_number = ((cls.random_int(0, 49) * 2) + 1)
    regon_digits = [int((voivodeship_number / 10)), (voivodeship_number % 10)]
    for _ in range(6):
        regon_digits.append(cls.random_digit())
    regon_digits.append(regon_checksum(regon_digits))
    return ''.join((str(digit) for digit in regon_digits))
