propagating exceptions.

context manager's __exit__ method's result while it's being converted to bool.

Fixes Issue #874900: after an os.fork() call the threading module state is cleaned
up in the child process to prevent deadlock and report proper thread counts
if the new process uses the threading module.

whose write() method installs another sys.stdout.

Backport of r64633

When an unfinished generator-iterator is garbage collected, PyEval_EvalFrameEx
is called with a GeneratorExit exception set.  This leads to funny results
if the sys.settrace function itself makes use of generators.
A visible effect is that the settrace function is reset to None.
Another is that the eventual "finally" block of the generator is not called.

It is necessary to save/restore the exception around the call to the trace
function.

This happens a lot with py3k: isinstance() of an ABCMeta instance runs
    def __instancecheck__(cls, instance):
        """Override for isinstance(instance, cls)."""
        return any(cls.__subclasscheck__(c)
                   for c in {instance.__class__, type(instance)})
which lets an opened generator expression each time it returns True.

And the problem can be reproduced in 2.5 with pure python code.

masked by a generic one with the message "unpack non-sequence".
 (backport from rev. 54480)

sq_ass_slice instead of the sq_slice slot.
 (backport from rev. 54139)