Add python 3.13, remove python 3.8 support

[jhamon]

Problem

Want to support python 3.13, drop support for python 3.8

Solution

Dependency changes:

• Remove httpx, explored in testing but not needed for the release
• Move pytest-asyncio from dependencies into dev dependencies
• Adjust grpcio and pandas (dev dependency) for 3.13 compatibility

Type of Change

• Bug fix (non-breaking change which fixes an issue)