fix: consume and report CryptographyDeprecationWarning, detect encrypted PKCS#8 keys

[ai-anant]

Summary

Addresses #11 (Consume specific warnings / errors and report accordingly).

Changes

• keychecker/core/key_analyzer.py
  • Wrap key loading and public-key serialization in warnings.catch_warnings(record=True) and collect CryptographyDeprecationWarning messages (e.g. the "SSH DSA key support is deprecated" warning emitted by public_bytes(Encoding.OpenSSH) when serializing DSA keys). Warnings are surfaced in the analysis result under a new warnings key instead of leaking to stderr as raw Python warnings.
  • Recognize BEGIN ENCRYPTED PRIVATE KEY (OpenSSL PKCS#8 encrypted format) headers in _analyze_encrypted_key, so such keys report type: pkcs8 instead of unknown.
• keychecker/utils/output.py — display library deprecation warnings (⚠️) in both human-readable formatters.
• tests/test_key_analyzer.py — new tests: DSA key analysis captures the deprecation warning without leaking it; encrypted PKCS#8 keys detected as pkcs8; normal RSA keys have empty warnings.

Verification

• pytest tests/ → 9 passed
• black --check keychecker/ tests/ → clean
• flake8 keychecker/ tests/ → clean
• Manual CLI check: analyzing a DSA key now prints ⚠️ SSH DSA key support is deprecated and will be removed in a future release in the output and produces no stderr warning trace; an OpenSSL genpkey -aes256 key reports Type: pkcs8.

Fixes #11