python: make sure SIGINT properly interrupts everything

[lidavidm]

We're going to need some finagling. Either:

• If we're on the main thread, punt long operations to a background thread that we join on. If we're interrupted in Python, then cancel the operation, join the thread, and re-raise the interrupt.
• If we're on the main thread, set a custom signal handler and spawn a background thread. If we're interrupted, the signal handler wakes the background thread via a self-pipe. The background thread then cancels the operation. The main thread restores the signal handler when we return to Python.