fix: keep deprecated altchars handling when decoding with padded=False

[Shaisolaris]

With validate=True and padded=False, the C extension rejects + or / in the input when custom altchars are given, while the pure Python fallback (and the Python 3.15 base64.b64decode it mirrors) still decode the data and emit the DeprecationWarning, because ignorechars was not specified:

import pybase64
pybase64.b64decode("+KKK", "-_", validate=True)                # b'\xf8\xa2\x8a' + DeprecationWarning
pybase64.b64decode("+KKK", "-_", validate=True, padded=False)  # binascii.Error: Non-base64 digit found

padded=False sets the internal ignorechars default, and that branch switched to the strict translate function. This change only switches to strict translation when the caller passed ignorechars, so an explicit ignorechars keeps raising as before (test_altchars_translation is unchanged).

Tests: added test_warning_data_dec_unpadded, the padded=False variant of test_warning_data_dec, including the 16 KiB slice case. Locally (Linux x86_64, Python 3.12, C extension built with all SIMD paths available up to AVX512VBMI):

• before: 14 of the 28 new cases fail (the C validate=True ones); the full tests directory gives 14 failed, 22088 passed, 58 skipped
• after: 22102 passed, 58 skipped

I did not run the sanitizer build, other platforms or Python 3.15. I left the changelog in the README alone in case you prefer to write that entry.

[codspeed]

Merging this PR will not alter performance

✅ 48 untouched benchmarks

---

_{Comparing Shaisolaris:fix-validate-unpadded-altchars (5f07b89) with master (4d37a46)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.58%. Comparing base (4d37a46) to head (5f07b89).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1075   +/-   ##
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  Coverage   99.58%   99.58%           
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  Files          11       11           
  Lines        1937     1950   +13     
  Branches      306      307    +1     
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+ Hits         1929     1942   +13     
  Partials        8        8           

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[mayeut]

Thanks for the PR.
I'm a bit curious: how did you come across this issue ? just a test case downstream or from a real use case ?