bpo-42615: Delete redundant jump instructions that only bypass empty blocks (GH-23733)

* Delete jump instructions that bypass empty blocks

* Add news entry

* Explicitly check for unconditional jump opcodes

Using the is_jump function results in the inclusion of instructions like
returns for which this optimization is not really valid. So, instead
explicitly check that the instruction is an unconditional jump.

* Handle conditional jumps, delete jumps gracefully

* Ensure b_nofallthrough and b_reachable are valid

* Add test for redundant jumps

* Regenerate importlib.h and edit Misc/ACKS

* Fix bad whitespace