fix pytest module discovery in CI

CI invokes the pytest console script directly (pytest tests/ -v), which
unlike `python -m pytest` does not add the repo root to sys.path. That
broke imports of the top-level data_processing/nightscout_client
modules from the test files. Add pythonpath = ["."] so it works
regardless of invocation style.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

pyproject.toml

@@ -0,0 +1,2 @@
+[tool.pytest.ini_options]
+pythonpath = ["."]