Add pyproject.toml packaging and pytest suite

- pyproject.toml: setuptools build, deps (requests), dev extra (pytest),
  pytest config with opt-in `live` marker
- tests/: 95 offline tests locking every series-ID encoding against
  known-good IDs (regression guard for the JOLTS/OES/ECI/productivity/QCEW
  fixes), plus a live API smoke test behind `-m live`
- README: install-as-package + test instructions; drop the now-resolved
  "no packaging"/"no tests" limitations
- gitignore build/test artifacts

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -2,3 +2,12 @@ config.py
 __pycache__/
 *.pyc
 .env
+
+# packaging / test artifacts
+build/
+dist/
+*.egg-info/
+.eggs/
+.pytest_cache/
+.venv/
+venv/