ci: don't run live-prod tests in CI; run pytest locally only

[martinkersner]

Summary

CI was failing only on the Python 3.14 job — not a 3.14 bug. The Test with pytest step was gated to if: matrix.python-version == '3.14', so 3.14 was the only version that actually ran the suite. Those tests hit live production endpoints and depend on prod data availability, so they fail nondeterministically in CI:

• DEX endpoints return 404 (removed upstream, see Remove deprecated DEX support and DEX tests (endpoints return 404) #89).
• /api/v1/premium returns an empty page for some filter combos, which the SDK turns into ValueError("no data found").

This PR removes the pytest step from .github/workflows/python-package.yml. CI now runs only black + flake8 across the 3.10–3.14 matrix and never touches the production API. The test suite is unchanged and stays runnable locally:

DATAMAXI_API_KEY=... python -m pytest tests/

Tests

• No tests run in CI by design (this is the point of the change).
• Locally, the suite still collects and runs against prod as before.

Known failures

• None introduced. The previously-failing 3.14 job is the failure this removes.
• The DATAMAXI_API_KEY repo secret is now unused by this workflow; left in place intentionally.

Closes #95