sys._current_exceptions() should return exceptions rather than exc_info tuples
The function sys._current_exceptions() returns a mapping of thread-id to an exc_info tuple representing that thread's handled exception. As part of the larger effort of moving away from exc_info to simple exception instances, we should change this mapping to have just the exception instance as value.
While this is a breaking change, I think we can do this without a deprecation period because this function is named with a leading _, and it is documented as "for internal and specialized purposes only".