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* Add `--encoding` option and default it to `utf-8`
* Drop support for Python < 3.6
* Fix issues found by pylint 2.11.1
(fixes [Debian bug 998574](https://bugs.debian.org/998574)):
* tests: Use `with` for `subprocess.Popen` calls
* Use `from ruamel import yaml`
* Replace `.format()` with f-strings