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

The `_calculate_checksum` method in `InternationalStandardSerialNumber`
was missing a final `% 11`, causing it to return 11 (an invalid
two-digit value) instead of 0 when the weighted digit sum happened to
be exactly divisible by 11.

Fixes #238.

Author: binggao1230

barcode/isxn.py

@@ -113,7 +113,7 @@ def _calculate_checksum(self):
             11
             - sum(x * int(y) for x, y in enumerate(reversed(self.issn[:7]), start=2))
             % 11
-        )
+        ) % 11
         if tmp == 10:
             return "X"
 

tests/test_checksums.py

@@ -46,3 +46,16 @@ def test_isbn13_checksum() -> None:
 def test_gs1_128_checksum() -> None:
     gs1_128 = get_barcode("gs1_128", "00376401856400470087")
     assert gs1_128.get_fullcode() == "00376401856400470087"
+
+
+def test_issn_checksum_zero() -> None:
+    # When the weighted sum is divisible by 11, the checksum should be 0,
+    # not 11. Regression test for issue #238.
+    issn = get_barcode("issn", "6727893")
+    assert issn.issn == "67278930"  # type: ignore[attr-defined]
+
+
+def test_issn_checksum_x() -> None:
+    # When (11 - weighted_sum % 11) % 11 == 10, checksum should be "X".
+    issn = get_barcode("issn", "0000006")
+    assert issn.issn == "0000006X"  # type: ignore[attr-defined]