Use upstream ruamel.yaml instead of ruamel_yaml

[mbargull]

Description

I'm currently looking at what packages on conda-forge have a dependency on the non-maintained ruamel_yaml package.

It'd be good if we could have this package depend on ruamel.yaml instead of ruamel_yaml, too.

NB: The latest conda-forge::ruamel_yaml is still at version 0.15.80 -- upstream released that version 5 years ago.

Conda-forge will stop (re-)building (the severly outdated) ruamel_yaml package;
tracking issue: conda-forge/ruamel_yaml-feedstock#107

xref: conda-forge/faunus-feedstock#42

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This should also add compatibility for ruamel.yaml=0.18.
ruamel.yaml=0.18 removed the top-level load, dump, etc. functions.
For details see https://pypi.org/project/ruamel.yaml/0.18.4/ .

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As indicated in the incomplete checklist below, the changes have unfortunately, not been tested by myself.
Feel free to cherry-pick these changes or push to this PR's branch directly (Allow edits by maintainers is enabled) if changes are needed.

Checklist

• make test passes with no errors

• the source code is well documented

• new functionality includes unittests

• the user-manual has been updated to reflect the changes

• I have installed the provided commit hook to auto-format commits (requires clang-format):

  ./scripts/git-pre-commit-format install

• code naming scheme follows the convention:

  Style	Elements
  PascalCase	classes, namespaces
  camelCase	functions
  snake_case	variables

[mlund]

Thanks for looking into this.

[mbargull]

Thanks for the extremely fast reaction :)