[3.14] GH-135171: Fix generator expressions one last time (hopefully)

[markshannon]

This PR adds a NULL check for tp_iternext to FOR_ITER to prevent the crash in #125038.
It then moves GET_ITER back to generation expression creation, thus fixing #135171 and avoids re-introducing #127682 by ensuring GET_ITER is never added twice.

I've chosen to make a new PR rather than reverting earlier ones as I want to keep all the new tests we've added, and it is probably easier to review than multiple reverts.

@hugovk @Yhg1s

• Issue: Generator expression behavior changed in 3.13.4 - it does not throw exception anymore #135171

[serhiy-storchaka]

Please restore also old tests.

I think this change should be made in main.

[markshannon]

I'll add back the old test.

This change can't be made in main due to the the changes to the way that iteration is implemented.

[serhiy-storchaka]

Then we need to rollback more changes in main. __iter__ should be called when the generator object is created.

[efimov-mikhail]

I'm not exactly sure about changes in codegen.c, but everything else LGTM.