Lazy import either 'anyio' or 'trio'

[lovelydinosaur]

Closes#626

example.py

importtrioimporthttpcoreasyncdefmain():
asyncwithhttpcore.AsyncConnectionPool() ashttp:
response=awaithttp.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. 🤷‍♂️

[lovelydinosaur]

@Kludex I'm going with the "yes these need to be separate, because we're being careful and strict about typing". Functionally it would work with a bit of overlap here, but I'd suggest let's keep things neatly separated.