test: enforce warnings-as-errors in pytest configuration

All 1040 tests already pass with `pytest -W error`; this commit codifies
that constraint in `pyproject.toml` so future regressions are caught
automatically without needing to remember the manual flag.

Closes #541

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -72,6 +72,9 @@ sqlite-utils = "sqlite_utils.cli:cli"
 requires = ["setuptools"]
 build-backend = "setuptools.build_meta"
 
+[tool.pytest.ini_options]
+filterwarnings = ["error"]
+
 [tool.flake8]
 max-line-length = 160
 # Black compatibility, E203 whitespace before ':':