Three real issues surfaced when running the suite with strict warnings:
1. src/core/format_standardize.py: ``datetime.utcfromtimestamp`` is
deprecated in CPython 3.12 and slated for removal. Replace with
``datetime.fromtimestamp(ts, tz=timezone.utc)``. Output for the
date-only format codes we use is byte-identical.
2. src/core/io.py: ``list_sheets`` leaked the openpyxl file handle by
returning ``xl.sheet_names`` from an unclosed ``pd.ExcelFile``.
Wrap in a ``with`` block so the FD closes deterministically — also
prevents the Windows-only "file is locked" repro path.
3. tests/test_corpus.py: ``TestXlsxPollution.workbook`` fixture
returned the bare ``pd.ExcelFile`` instead of yielding + closing.
Convert to a yield-and-finally pattern so the class-scoped handle
isn't leaked across the whole test file.
Also harden pytest.ini's warning policy: escalate
``ResourceWarning`` from ``src`` to an error, alongside the existing
``DeprecationWarning`` rule. Third-party warnings stay filtered — we
can't fix pandas/openpyxl/streamlit churn from here.
All 1916 tests pass under the strict filter; full and split runs
(``pytest``, ``pytest -m 'not gui'``, ``pytest -m gui``) all clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Michael
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