Modernize and fix CI

[oremanj]

Code changes:

• Port from MultiError to ExceptionGroup, thus supporting (and requiring) Trio v0.22.0+.
• Drop support for 3.6 (required by ExceptionGroup support).
• Required by a new test in 3.12 but a good idea anyway: Route exceptions from asyncio callbacks through the default exception handler.
• Required by exception change above: Stop using StopAsyncIteration to indicate that the trio-asyncio loop should exit. Instead use a new TrioAsyncioExit exception, which derives from SystemExit and thus will pass through asyncio internals.
• Required by exception change above: Actually raise uncaught exceptions out of open_loop() when using an async loop, instead of just claiming to. Raising them out of default_exception_handler doesn't work because asyncio catches it. Instead we now create a new task in the loop nursery to raise the exception.
• Noticed because of exception change above: Fix a race condition where add_reader or add_writer on a socket object (rather than fd) could produce an uncaught exception if the reader/writer were unregistered and the socket closed inside the same reader/writer callback.
• Monkeypatch asyncio.current_task on 3.12+ so that it looks at the current_loop contextvar instead of just the C-level running loop cache. This is now necessary because current_task is now implemented in C and thus bypasses our monkeypatches by default.
• Clear Trio state on fork (required by new tests in 3.12).

Test changes:

• Test on 3.10, 3.11, 3.12-dev, and recent pypy.
• Move setup.cfg and pytest.ini into pyproject.toml.
• Update the pytest plugin that runs stdlib asyncio tests so it doesn't use deprecated pytest features.

Closes #119, #120.

[dhirschfeld]

bump!

Is there anything preventing this being merged?

[oremanj]

Is there anything preventing this being merged?

Just the fact that no one has approved it.

[dhirschfeld]

Since @agronholm has reviewed I'll let him approve if he's not too busy...

[agronholm]

I may be a little out of my depth here but I'll see if I can give this a proper review over the weekend.