Use custom YAML subclass to be compatible with ruamel_yaml>=0.17

[mweinelt]

Fixes a DeprecationWarning for ruamel_yaml>=0.17

/build/source/whipper/test/test_result_logger.py:167: PendingDeprecationWarning:
dump will be removed, use

  yaml=YAML(typ='unsafe', pure=True)
  yaml.dump(...)

instead
  ruamel.yaml.dump(
/nix/store/lf2qpwvjp3gxgrjkcvcw18d3zlrxsif8-python3.8-ruamel.yaml-0.17.9/lib/python3.8/site-packages/ruamel/yaml/main.py:1364: PendingDeprecationWarning:
dump_all will be removed, use

  yaml=YAML(typ='unsafe', pure=True)
  yaml.dump_all(...)

instead
  return dump_all(
F
======================================================================
FAIL: testLogger (whipper.test.test_result_logger.LoggerTestCase)
----------------------------------------------------------------------

ℹ️ Subclasses ruamel.yaml.YAML as whipper.common.yaml.YAML and moves common values for width and default_flow_style into whipper.common.yaml.YAML. RoundTripDumper, as I understand it, is the default dumper.

✔️ Tested with ruamel_yaml 0.16.13 and 0.17.9

❓ Not sure about the license header for whipper.common.yaml, since this is mostly copy/paste from the ruamel_yaml documentation (https://yaml.readthedocs.io/en/latest/example.html#output-of-dump-as-a-string)

[github-actions]

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[JoeLametta]

Looks good, thanks! Don't know what to do about the license thing.

EDIT: This change also seems to fix a test failure which started manifesting with ruamel.yaml 0.17.5 (later versions too).

[JoeLametta]

Merged, thanks!