Add optional sanitization of illegal XLSX characters on export (fixes #370)

[apoorvdarshan]

Summary

Exporting a Dataset (or Databook) whose string cells contain control characters that are illegal in XLSX crashed with openpyxl.utils.exceptions.IllegalCharacterError and gave callers no way to produce a file (issue #370):

from tablib import Dataset
data = Dataset()
data.append(('\x1f',))
data.export('xlsx')  # IllegalCharacterError

These are the characters matched by openpyxl's ILLEGAL_CHARACTERS_RE ([\000-\010]|[\013-\014]|[\016-\037]).

Fix

Added an optional sanitize_illegal_chars parameter to export_set() and export_book() in src/tablib/formats/_xlsx.py, following the design discussed by the maintainers on the issue (a string replacement char, with None/False meaning "raise", and substituting only when openpyxl actually raises rather than regex-scanning every cell):

• Default (None, or False): behavior is unchanged — the IllegalCharacterError still bubbles up, so test_xlsx_wrong_char and existing callers are unaffected.
• A string value: illegal characters in string cell values are replaced with it via ILLEGAL_CHARACTERS_RE.sub(...). Use "" to strip them (an empty string is treated as a valid replacement, not as "raise").

The substitution happens inside the existing try around cell.value = col, so the common clean path pays no extra cost.

data.export('xlsx', sanitize_illegal_chars='')   # strip
data.export('xlsx', sanitize_illegal_chars='?')  # replace

Tests

Added to tests/test_tablib.py:

• test_xlsx_export_illegal_char_raises_by_default — the exact issue repro; confirms the default still raises IllegalCharacterError.
• test_xlsx_export_set_sanitize_illegal_chars — strip ("") and replace ("?") paths for export_set.
• test_xlsx_export_book_sanitize_illegal_chars — the replacement path for export_book.

The two sanitize tests fail on pristine master (the parameter did not exist) and pass with the fix. All existing xlsx tests, including test_xlsx_wrong_char, remain green. ruff passes on the changed files. Docs (docs/formats.rst) and AUTHORS updated.

Disclosure: prepared with AI assistance; reviewed and verified locally.