support trio async event loop via anyio

[SecretiveShell]

This PR migrates away from direct asyncio usage in favour of AnyIO.

The advantages of this include support for using exa-py with trio or curio, and more reliable structured concurrency.

Additional notes:

• Explicitly added anyio to pyproject.toml for clarity, though it was already an indirect dependency via httpx. The lack of a pinned version matches their pyproject.toml in order to not impose any extra restrictions on dependency resolution.
• All unit tests pass with the exception of the known failing test tracked in issue Unit tests failing - test_research_task_live: 'ResearchClient' object has no attribute 'create_task' #118, and all external interfaces are identical.