Fix failing CI tests (ffmpeg on apple silicon, PySide2 problems)

[GenevieveBuckley]

In this PR:

1. brew install ffmpeg for Apple Silicon Mac arm64 runners on github actions, since imageio-ffmpeg does not provide ffmpeg binaries for Apple Silicon ffmpeg not included in wheel for MacOS M1 imageio/imageio-ffmpeg#71
2. Restrict PySide tests to Mac Intel (not Apple Silicon), and for all operating systems python>=3.10. PySide is not available for these configurations.
3. Drop python 3.8, and add python 3.12 to the CI test matrix

[jni]

Nice, thanks @GenevieveBuckley!