The `# -*- coding: utf-8 -*-` headers were useful for Python 2, and
aren't needed for Python 3 where UTF-8 is the default.
yamllint support of Python 2 was dropped in early 2021, see commit
a3fc64d "End support for Python 2".
Let's drop these headers.
To test yamllint as a module, tests run commands like
`python -m yamllint`. But some environments (like continuous integration
of Debian or CentOS) don't always include the `python` executable (they
use `python3` instead).
Let's dynamically detect the Python executable path.
