Python Unified SDK 8.0.0

This major release replaces ecdsa with the cryptography library and removes Python 3.9 support.

What's Changed

• [#201] Remove Python v3.9 support
• [#221] Replace ecdsa with cryptography for key generation and signing
• Bump pydantic from 2.11.9 to 2.12.5
• Bump black from 25.12.0 to 26.1.0
• Bump cryptography from 46.0.4 to 46.0.5
• Security: Fix untrusted input vulnerability in release workflow - 8.0.x

Full Changelog: https://github.com/bitpay/python-bitpay-client/compare/7.0.8...8.0.0