OpenImageIO 2.1.8.1-RC2

This is the second and hopefully final release candidate for the IMMINENT transition of 2.1 to be the supported release branch. I plan to retag it as the final release on 8 December. The API/ABI is hereby and irrevocably frozen; subsequent 2.1.x releases should all be fully back compatible for both API and ABI. The only changes since RC1 are a few minor build/warning fixes, and switch of the embedded "fmt" string formatting library to a newer version. Full release notes will be published on 8 Dec when the final release is announced (or you can check the CHANGES.md that comes with it).