Fix Python 3.12+, Tornado 6.3+ event loop deprecation warnings

[dwskoog]

Tornado deprecated and changed semantics around their IOLoop in major ways in 5.0, 6.2 and 6.3:

• in 5.0, they removed the distinction between "current" and "instance" loops and deprecated IOLoop.*_instance as they are just aliases for the analogous _current call.
• in 6.2, they deprecated the IOLoop methods for manipulating the active loop and recommended using the builtin asyncio.set_event_loop call to establish the operant loop. Note that in Python 3.13+ this is bad advice as written because asyncio.set_event_loop requires the loop instance to be an asyncio.AbstractEventLoop and Tornado's IOLoop is not. If you want to manually install it, the correct call seems to be asyncio.set_event_loop(my_io_loop.asyncio_loop) but this largely unnecessary as IOLoop is mostly just a wrapper around the default loops and gets things in the right place when its API is used.
• In 6.3, they changed the default value of make_current on the IOLoop constructor to True instead of False so calling IOLoop() greedily tries to establish itself as the loop of the current thread. This invalidates the intended semantics behind core.py:get_io_loop for binding to the new thread.

Once I removed the deprecated calls to Tornado APIs and fixed the default parameters (all test plumbing except the noted get_io_loop), this change in asyncio itself popped up:

• Python 3.12 deprecated summoning a new loop via get_event_loop and in 3.14 that is a runtime error. This required making test_core.py:test_timed_window_timedelta into a coroutine and running it via pytest-asyncio explicitly to get a loop in place as calling get_io_loop(asynchronous=True) calls IOLoop.current() which is just a wrapper around asyncio.get_event_loop()

[martindurant]

Thank you for the deep digging here! From your description, I was worried it would be more involved code changes than it actually is. One thing that you noticed here, and I think it indeed in the documentation, is that asynchronous=True should only be used from within asynchronous code; and we could enforce this. fsspec faces a similar situation.

I had spent some effort in the past converting tornado->asyncio, but there has been a decent amount of leftover legacy code left in the package. There is no reason to have tornado in here at all these days.