Remove dead [tool.sqla_testing] from pyproject.toml (setup.cfg cannot migrate)

[laughingman7743]

Summary

pyproject.toml contains a [tool.sqla_testing] block (lines 129-131) that duplicates setup.cfg, but SQLAlchemy's test plugin never reads it — it is dead configuration and should be removed. Conversely, setup.cfg cannot be migrated to pyproject.toml today, so it must stay.

Investigation

SQLAlchemy's testing plugin hardcodes reading setup.cfg / test.cfg via configparser and has no TOML/pyproject support:

# sqlalchemy/testing/plugin/plugin_base.py (2.0.46; identical on upstream main)
def read_config(root_path):
    global file_config
    file_config = configparser.ConfigParser()
    file_config.read(
        [str(root_path / "setup.cfg"), str(root_path / "test.cfg")]
    )

• requirement_cls is read from file_config.get("sqla_testing", "requirement_cls") (CLI override --requirements exists).
• profile_file is read from file_config.get("sqla_testing", "profile_file") and has no CLI override and no default — it can only come from setup.cfg/test.cfg. Deleting setup.cfg breaks just test sqla / sqla-async with configparser.NoSectionError.
• Verified on the installed SQLAlchemy 2.0.46 and on upstream main — no released or unreleased version reads this config from pyproject.toml.
• Nothing else in the repo (justfile, CI workflows, docs) references setup.cfg; it exists solely for SQLAlchemy's test plugin.

Action items

• Remove the never-read [tool.sqla_testing] block from pyproject.toml (lines 129-131) — no behavioral change.
• Add a comment at the top of setup.cfg explaining it exists solely for SQLAlchemy's compliance test plugin (sqlalchemy.testing.plugin.plugin_base.read_config) and cannot move to pyproject.toml until upstream SQLAlchemy supports it.