Lazy import either 'anyio' or 'trio'

[lovelydinosaur]

Closes #626

example.py

import trio
import httpcore

async def main():
    async with httpcore.AsyncConnectionPool() as http:
        response = await http.request("GET", "https://www.example.com/")
        print(response)


trio.run(main)

$ pip install httpcore, trio
$ pip uninstall anyio
$ python example.py
<Response [200]>

Turns out this doesn't fix the #588 conflict, which is still triggered by the usage of @pytest.mark.anyio in the test suite. 🤷‍♂️