Message 344386 - Python tracker

> Can we at least switch to PyLong_CheckExact?

This is a behavior change and as such should be preceded by a period of warning.

If we go this way I propose to add a FutureWarning for int subclasses with overridden __index__.

As for turning the deprecated behaviour into TypeErrors, we added yet few deprecation warnings in 3.8. Would not be better to turn all of them into TypeErrors at the same time?