This is a follow-up to D13199 for multiline strings that were not processed by `flynt`
Also replace `repr(thing)` by `f"{thing!r}" which is more compact.
I suspect most of these are uneccessary, as `str(thing)` and `repr(thing)` are usually the same for objects that only define a `__repr__` method and not `__str__` method. But there are too many of them to check them all.
Depends on D13212