Accept uppercase X as the ISBN-10 check digit

[vitalivo]

This replaces #31, which was accidentally closed and its source fork deleted. The implementation is unchanged; the original discussion and reviews remain linked there.

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is_isbn_10('080442957X') and is_isbn('080442957X') currently return False because the checksum loop tries to convert the final X with int().

Treat an uppercase X in the tenth position as 10, as specified in the Library of Congress ISBN structure documentation. Other positions and lowercase x remain invalid, and the ISBN-13 calculation is unchanged.

Tests cover two valid X-ending ISBNs, hyphen normalization, disabled normalization, an incorrect checksum, and misplaced/repeated X characters. Four assertions fail before the fix; all 409 tests pass with python -m unittest discover on Python 3.13.