Add support for backend="anyio"

[lovelydinosaur]

Builds on #169, with a minimal as possible change footprint.

Last hurdle to clear would be switching out the cases where I've used backend="anyio" to be parameterized to use both backend="auto"|"anyio". The unasync.py script will need to strip out that parameterisation for the sync case, which needs to just always use backend="sync".

I'm really keen on us also using anyio to slim down any gnarly in the test suite that it can help us address, but we should treat that as an entirely isolated issue. (Using anyio to run our tests is an independent concern to what backend implementation we want to use.)

[lovelydinosaur]

Also, I think this change bumps up our minimum required trio version, which we're currently just being lax about.

(I happened to be running locally with trio 0.14.0, which passed our existing test suite, but failed with backend="anyio", but upgrading to trio 0.16.0 resolved this.)

[agronholm]

This won't test the explicit backends used by the "auto" backend though, does it?

[lovelydinosaur]

This won't test the explicit backends used by the "auto" backend though, does it?

At this point in time, yup. It's only testing "anyio", eg...

asyncdeftest_http_request() ->None:
asyncwithhttpcore.AsyncConnectionPool(backend="anyio") ashttp: # Explicitly testing against anyio only.method=b"GET"url= (b"http", b"example.org", 80, b"/")
headers= [(b"host", b"example.org")]

The last step here would be to parameterise it so that both backend="anyio" and backend="auto" end up being tested. (And making a little nudge in unasync.py so that the sync case is only parametereized with a single case, for backend="sync")

[florimondmanca]

Closing tentatively since it looks like the suggestions from this PR were integrated into #169. :-)