"asynchronous generator is already running" when using simulator process on Python <= 3.12

(Originally posted to #1590, but in a new issue for visibility and tracking.) I'm getting RuntimeErrors during simulation when I use a background process that does not complete by the time the simulation ends. Here is a small example: ````.py import amaranth as am import amaranth.sim async def deadline_checker(ctx): await ctx.tick().repeat(100) raise RuntimeError('deadline') m = am.Module() counter = am.Signal(8) m.d.sync += counter.eq(counter + 1) sim = am.sim.Simulator(m) sim.add_process(deadline_checker) sim.run() ```` Under Python 3.13.5, this works fine. With Python 3.12.8, I get ```` Exception ignored in: <coroutine object deadline_checker at 0x7f433c8c9f00> Traceback (most recent call last): File "/home/agrif/devel/fpga/simulator_aclose.py", line 7, in deadline_checker File "/home/agrif/devel/fpga/local/nara-env-312/lib/python3.12/site-packages/amaranth/sim/_async.py", line 414, in repeat RuntimeError: aclose(): asynchronous generator is already running ```` While reading #1590, I was confused by why `aclose(...)` is grumpy, so I dug around. I don't get much from this, but maybe someone else will (or me, later). * [Python documentation for aclose](https://docs.python.org/3/reference/expressions.html#agen.aclose) * [first CPython commit with the new behavior (no RuntimeError)](https://github.com/python/cpython/commit/e5c699280deac076cddfef37c8af917a550f6ac3)