Make install-requires/tests-require pyproject.toml-aware (closes #38)

[thorwhalen]

Summary

isee install-requires / isee tests-require — and therefore the
install-packages action's dependency-files path — only understood
setup.cfg, hard-failing on pyproject.toml-only repos with
RuntimeError: No file with name "setup.cfg" exist .... This silently broke CI
for any pyproject-only repo whose ci.yml still passed a .cfg.

Fixes #38.

What changed

• isee/pip_utils.py — new _metadata_source() resolver: prefer
  pyproject.toml when it has a [project] table, otherwise fall back to
  setup.cfg.
  • install_requires → [project].dependencies (else [options] install_requires).
  • tests_require → [project.optional-dependencies] (first present of
    testing/test/tests/dev, overridable via test_extras; else
    [options] tests_require).
  • Empty deps → logged no-op (No <kind> packages to install); only a total
    absence of metadata raises, now with an actionable message.
  • New side-effect-free resolve_install_requires / resolve_tests_require
    expose resolution without invoking pip (and back the tests).
  • Legacy read_setup_config / extras_require / install_extras kept.
• actions/install-packages/action.yml — the Install Dependencies step now
  triggers on *.toml as well as *.cfg; input description updated.
• pyproject.toml — tomli fallback dependency for Python 3.10 (tomllib
  is 3.11+).
• tests/test_pip_utils.py — covers pyproject resolution, custom extras,
  empty-deps no-op, pyproject-preferred-over-cfg, cfg fallback when pyproject
  carries only tool config, and the clear error when neither file exists.

Testing

pytest tests/test_pip_utils.py → 9 passed. Full suite: only the 3
pre-existing local_cli --bind failures remain (unrelated to this change;
they also fail on master).