Use pip's vendored truststore to avoid conflicts

Resolves compatibility issues with modern pip by using pip's own vendored
truststore instead of requiring a separate truststore installation. This
eliminates SSL context conflicts while ensuring all libraries benefit
from system certificates.

Key changes:
- Import truststore from pip._vendor instead of standalone package
- Change dependency from truststore>=0.8.0 to pip>=24.2
- Add fallback logic for older pip versions (auto-upgrades to pip 24.2+)
- Remove pip version detection logic (no longer needed)
- Update README to reflect new vendored truststore approach

Benefits:
- No more recursion errors when pip and package both use truststore
- Automatic pip upgrade ensures vendored truststore availability
- Both pip and requests/other libraries use same truststore instance
- Maintains compatibility across all pip versions via auto-upgrade

Testing confirms pip installations work correctly while requests and
other SSL libraries receive system certificate support.

🤖 Generated with [Claude Code](https://claude.ai/code

)

Co-Authored-By: Claude <noreply@anthropic.com>