Rewrite f"x={x!r}" to f"{x=} on Python 3.8+

[Zac-HD]

It would be lovely to have this built into pyupgrade!

For HypothesisWorks/hypothesis#3657 I did this with a regex replacement r"(.+?)=\{\1!r\}" -> r"\{\1=\}", which in principle is super inefficient but in practice worked just fine. Applying it only to f-string tokens would be a nice precision and safety improvement, which I'd appreciate as part of my autoformatting stack 🙂