Several areas for attention:
* Changes to the test suite probably should not be made
  The user doesn't benefit in any way and you risk
  unintentionally breaking the test suite in a way that
  isn't obvious (we have no tests for the tests themselves
  so changing tests is like refactoring without a safety net).
* Some of the exception handling in IDLE needs to catch
  all exceptions (i.e. SystemExit and KeyboardInterrupt
  are supposed to display tracebacks and not terminate
  IDLE itself).
* Changing the exceptions in threading.py is worrisome.
* The Pdb debugger may have legitimate reasons to catch
  all exceptions (like IDLE, we want don't want to 
  terminate the debugger itself).

In other words, many of the proposed changes should not
be made.  

For the rest, be careful to not change semantics
unintentionally and consider adding tests if you think
there is a real bug.   In cases where there is doubt
about the right thing to do, consider assigning the
original author or maintainer of the code.