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>

tests/conftest.py

@@ -0,0 +1,9 @@
+"""Shared test fixtures and path setup.
+
+Lets the suite run straight from the repo (`pytest`) without installing the
+package, while also working fine when bls_client is pip-installed.
+"""
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(__file__)))