fix: ISSN checksum returns 11 instead of 0 when weighted sum is divisible by 11

[gaoflow]

Summary

Fixes #238.

The _calculate_checksum method in InternationalStandardSerialNumber was missing a trailing % 11. When the weighted digit sum is exactly divisible by 11, the formula computes 11 - 0 = 11 — an invalid two-digit value — instead of (11 - 0) % 11 = 0.

Before:

tmp = (
    11
    - sum(x * int(y) for x, y in enumerate(reversed(self.issn[:7]), start=2))
    % 11
)

After:

tmp = (
    11
    - sum(x * int(y) for x, y in enumerate(reversed(self.issn[:7]), start=2))
    % 11
) % 11

Minimal reproduction:

from barcode import ISSN
print(ISSN("6727893").issn)  # was "672789311", now correctly "67278930"

Two regression tests added to tests/test_checksums.py: one for the zero-checksum case and one confirming the X checksum case still works.

This pull request was prepared with the assistance of AI, under my direction and review.

[WhyNotHugo]

Sounds correct to me.

[WhyNotHugo]

Thanks!