Update pyproject.toml -- new build backend, description, license metadata

[gvanrossum-ms]

• Switch build backend to uv_build in pyproject.toml and remove setuptools-specific toml entries.
• Update description in pyproject.toml.
• Update license metadata in pyproject.toml.
• Remove logfire and pydantic_ai from hard dependencies in pyproject.toml.
  • This implies that query.py --logfire and typeagent.aitools.utils.setup_logfire() don't work unless you install logfire.
  • It also implies that typeagent.aitools.utils.make_agent() doesn't work unless you install pydantic-ai-slim[openai].
• Tweak utils.make_agent() to have all pydantic_ai imports inside it. This compromises the return type, but I don't care.