+1 from me for the general idea.

One subtlety with the draft implementation is that an Initialize/Finalize cycle doesn't reset the counter, which:

1. Increases the chance of counter overflow (while admittedly still leaving it incredibly low)
2. Means you still can't readily check whether the current interpreter is the main interpreter (i.e. the one created automatically in Py_Initialize)

What do you think about resetting the counter back to 1 in Py_Initialize?