Migrate to uv and upgrade vulnerable development dependencies

[OliverSherouse]

What changed

• Migrated project metadata, dependency management, and packaging from Poetry to uv.
• Added a committed uv.lock and switched the pure-Python package to uv_build.
• Upgraded the Dependabot-flagged development dependencies: ipython and pytest.
• Replaced mypy with pinned ty 0.0.64.
• Declared Python 3.10+ support, dropping only EOL Python versions.
• Updated test and publish workflows to use pinned uv and current GitHub Actions setup.

Validation

• uv build --no-sources succeeds locally.
• GitHub Actions successfully runs uv sync --locked and uv run ty check bls on Python 3.10, 3.11, and 3.12.
• The integration tests currently fail because the repository's BLS_API_KEY secret is rejected by the BLS API (REQUEST_NOT_PROCESSED). This is independent of the migration; replace that secret with a valid key to restore those tests.