Try fixing macos build. 2to3 from 2.7 is not really used and is interfering with the installation of Python 3.9, on which one of ffmpeg's dependencies depends.
* ci: migrate to GitHub Actions * ci: linux-mingw: use lief for parsing PE files * ci: fix left-over issues with clang-format check * ci: workaround libc++ issue on macOS * appveyor: remove build scripts * README: add GitHub Actions badge and ... remove Travis CI and Appveyor badges