Make pdb on Windows interruptible

[itamarst]

This is the final fix for ipython/ipython#10516

Without the fix:

1. The test fails.
2. If you open Jupyter on Windows, run pdb.set_trace(), and then in same cell run some other code while pdb is still waiting for input, the interrupt button won't work.

With the fix:

1. The test passes.
2. pdb.set_trace() is finally interruptible on Windows, even in the scenario described above.

[itamarst]

@blink1073 one final PR and pdb is interruptible on Windows!

[blink1073]

Thanks! Because this changes a public API I marked it for the next minor release.