fix(finance): validate the ISIN check digit (#440)

[apoorvdarshan]

Summary

Fixes #440 (the ISIN portion). validators.isin() accepts any invalid ISIN check digit.

>>> import validators
>>> bool(validators.isin("US0378331005"))   # Apple, genuinely valid
True
>>> bool(validators.isin("US0378331004"))   # same but wrong check digit
True                                          # <-- should be False

Root cause

_isin_checksum() computes a per-character val and doubles it for odd positions, but never accumulates it into check:

check, val = 0, None
for idx in range(12):
    ...
    if idx & 1:
        val += val      # doubles val, but val is then discarded
return (check % 10) == 0  # check is always 0 -> always True

So the checksum is never actually validated — every 12-character string with a two-letter country code and a trailing digit passes. (_cusip_checksum and sedol accumulate correctly and are unaffected.)

Fix

Implement the real ISIN algorithm: expand letters to digits (A=10 … Z=35), keep digits as-is, then run the Luhn checksum over the resulting digit string. Also enforce that the country code (first two chars) is alphabetic and the check digit (last char) numeric, so lowercase/malformed inputs are rejected (as the issue requests).

Verification

• Validated against python-stdnum's ISIN validator: my implementation agrees on every case tested (real ISINs for Apple/Microsoft/BAE/Alphabet/Bayer/etc. → valid; wrong-check-digit variants → invalid).
• This cross-check also revealed the existing test fixture JP000K0VF054 was itself an invalid ISIN (correct check digit is 5); it only passed before because the checksum was never computed. Corrected to JP000K0VF055 and added wrong-check-digit fixtures.
• New tests fail on master and pass with this change. tests/test_finance.py: 31 passed (incl. doctests), up from 21.
• ruff check and ruff format --check clean on the changed files.
• The 17 pre-existing tests/crypto_addresses/test_eth_address.py failures are unrelated (they fail identically on master).

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Disclosure: prepared with AI assistance; reviewed and cross-checked locally (incl. against python-stdnum).