B016: Warn when raising f-strings

[FozzieHi]

Warns when raising f-strings which I think matches the check's current description.

[cooperlees]

Thanks. Surprised this isn't a Python error, but neither are non f-strings so thanks for adding this! Wonder how many it will find around the world.

I wish we somehow had stats of all this plugin finds!

[jakkdl]

I wish we somehow had stats of all this plugin finds!

There's definitely ways of estimating those numbers, estimating the baseline prevalence of an error in projects without bugbear and multiplying by [estimated] number of flake8-bugbear installs. Or checking the number of hits on github search for the invalid pattern before and after implementing a new check. But that will underestimate the impact, as it only checks in pushed code and not thorny-bug-that-takes-much-longer-to-find-without-bugbear-but-will-get-fixed.