pyupgrade: drop redundant `open()` modes On one hand, this gets rid of some py2 cruft (like `U` and `t` modes). OTOH, I like being explict. But there's no way to disable individual transformations (and this change is part of `_fix_tokens()`, which is even harder to selectively use locally), and I'd rather have less noise when using `pyupgrade`. Warning: `contrib/synthrepo.py` appears not to have been updated to py3, as it's passing str to `error.Abort`. But since it was opening the file in unicode on py2, this change is a no-op.