Add pytest test suite and replace pylint with ruff

[hyperfinitism]

Summary

Add a comprehensive test suite and modernize the CI linting/formatting pipeline.

Tests (tests/)

• Add 116 tests covering the library and example CLIs:
  • test_types.py — pysnputils.types (helper functions, enums, bitfield types, AttestationReport parsing)
  • test_fetch.py — pysnputils.fetch (KDS URL generation, fetch functions with mocked HTTP)
  • test_verify.py — pysnputils.verify (certificate chain validation, report signature verification with real ECDSA-P384 signing)
  • test_examples.py — examples/*.py CLI scripts (argument parsing, output format, mocked I/O)
• Add conftest.py with shared fixtures and helpers (synthetic report builder, certificate generators)
• Add .github/workflows/pytest.yml CI workflow (Python 3.11 / 3.12)

Linter / Formatter: pylint → ruff

• Remove .pylintrc and .github/workflows/pylint.yml
• Add [tool.ruff] configuration in pyproject.toml
  • Rule sets: E, W, F, I, UP, B, SIM, PL
  • Migrated pylint ignore rules (too-many-arguments, too-many-locals, etc.)
• Add .github/workflows/ruff.yml (ruff check + ruff format --check)
• Fix all lint/format errors across existing source files

Other

• Lower minimum supported Python version from 3.12 to 3.11 (requires-python, pytest matrix)
• Replace pylint with ruff in [project.optional-dependencies]

Tests

• local ruff check . — all checks passed
• local ruff format --check . — all 13 files formatted
• pytest tests -v — 116/116 passed
• Confirm CI workflows pass on GitHub (pytest, ruff, spdx)