Use Hypercorn for UDS tests

[florimondmanca]

Follow-up to #205

Use hypercorn for UDS tests. Had to refactor HypercornServer a bit to remove the host/port options, and use bind in all cases.

We can't ditch uvicorn entirely because hypercorn is 3.7+ only, so we only install and use uvicorn on 3.6.

The great part here is that we now use the Server interface across all tests.

[florimondmanca]

We've got a flake.

 > assert backend.pop_open_tcp_stream_intervals() == [ pytest.approx(0, abs=1e-3), # Retry immediately. pytest.approx(0.5, rel=0.1), # First backoff. pytest.approx(1.0, rel=0.1), # Second (increased) backoff. ]E assert [0.0010938644...1297950744629] == [0 ± 1.0e-03,...1.0 ± 1.0e-01]E At index 0 diff: 0.0010938644409179688 != 0 ± 1.0e-03E Use -v to get the full diff

[JayH5]

Nice!