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Issue 29988 covers the fact that with the default SIGINT handler installed, a poorly timed Ctrl-C can lead to context managers failing to even start running their __(a)exit__ methods, let alone complete them.
For the asynchronous case, the problem is even worse, as the *event loop* may be interrupted at arbitrary points if the default SIGINT handler is left in place.
To handle this robustly, it's desirable to make it easy to switch event-driven programs over to cooperative Ctrl-C handling by installing an asyncio SIGINT handler while the event loop is running, rather than leaving the default SIGINT handler in place.
(Note: while installing a cooperative SIGINT handler will enable more robust event-loop managed resource cleanup, it will have the downside that Ctrl-C won't be able to interrupt a coroutine that has incorrectly blocked the main thread)

Some prior discussion on the old asyncio tracker:
 https://github.com/python/asyncio/pull/305#issuecomment-168714572
 https://github.com/python/asyncio/issues/341 

Issue 31388 is also potentially relevant here, as registering a signal handler for SIGINT isn't sufficient to cover all potential cases were Py_AddPendingCall gets called. In particular, the tests for issue 29988 use Py_AddPendingCall to emulate Ctrl-C, rather than relying on SIGINT actually being raised.

As per Nathaniel's comments on issue 29988 and 31388, doing this robustly relies on:
1. the event loop being able to reliably guard itself and __aexit__ method implementations against interrupts (issue 31388)
2. "async with" statements ensuring that if the frame resumes after calling __aenter__, then it will also call __aexit__ (as was done for synchronous with statements in issue 29988)