Ionit

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* Support multiple templates folders (i.e. allow specifying `--templates`
multiple times) (2)

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* Make YAML files free of complaints from yamllint
* Update my email address to posteo.de

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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

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* Check import definition order with isort
* Fix code format for older black versions
* Add test case for black code formatter
* Fix YAMLLoadWarning with PyYAML 5.1+
* Add support for ruamel.yaml
* `setup.py`: Add PyYAML as dependency

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* Address issues found by pylint 2.6.0
(fixes [Debian bug 971138](https://bugs.debian.org/971138))
* Blacken Python code

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* Run `ionit.service` before `ifupdown-pre.service`