Fix monkeypatch.setattr() teardown crash on failed setattr

[worksbyfriday]

Fixes #14161.

When monkeypatch.setattr(target, name, value, raising=False) is called on a target that doesn't support attribute setting (e.g. None or other immutable objects), the underlying setattr() raises AttributeError. But the undo stack entry was appended before the setattr() call, leaving a stale (target, name, NOTSET) entry. During teardown, undo() then tries delattr(target, name), causing a second AttributeError crash.

Fix: Move the _setattr.append() call after the setattr() call, so only successful attribute modifications get undo entries.

# Before: crashes at teardown
something_or_none = None
monkeypatch.setattr(something_or_none, 'do_something', fake, raising=False)
# → setattr fails, but undo stack has stale entry
# → teardown: delattr(None, 'do_something') → AttributeError

# After: teardown is clean
# → setattr fails, undo stack is empty
# → teardown: nothing to undo

[RonnyPfannschmidt]

looks good, thanks

if you like you can use

with monkeypatch.context() as ctx:
       with pytest.raises(...):
          ctx.setattr(...)

to pin down the monkey-patch used to something very constrained