Add support for YAML data files in load_data [RHELDST-4904]

[crungehottman]

The load_data method will accept both YAML and JSON files, but will
load the data.yaml file by default.

This change enables cdn-definitions-private to cease maintaining
two sets of data, in both YAML and JSON forms -- only data.yaml
files will be provided and maintained. Those who wish to use
the cdn-definitions-private data will be able to do so without
a manual conversion of the YAML data file into a JSON file.

[rohanpm]

Only complaint is about adding the pathlib dep.

[rohanpm]

Have you verified that this displays correctly on the rendered docs?

The indent in general is meaningful. This commit is changing the string from:

Returns:
    list[:class:`~PathAlias`]
        A list of aliases relating to RHUI paths.

to:

Returns:
list[:class:`~PathAlias`]
    A list of aliases relating to RHUI paths.

Doc strings are processed via this tool: https://www.sphinx-doc.org/en/master/usage/extensions/napoleon.html

And in the examples there you can see that the docs for return value are always indented under "Returns", so I have a doubt whether this change is correct.
I'm not sure if the doc string parser is still going to process this the same way as it did before...

[rohanpm]

Is it black tool reformatting this?

[rohanpm]

I see now there's a separate commit in the PR saying that it's coming from black.

I checked this locally and I'm a bit confused actually, as both before and after this change sphinx generates warnings:

/home/rmcgover/src/cdn-definitions/.tox/docs/lib/python3.8/site-packages/cdn_definitions/_impl/__init__.py:docstring of cdn_definitions._impl.rhui_aliases:3: WARNING: Unexpected indentation.
/home/rmcgover/src/cdn-definitions/.tox/docs/lib/python3.8/site-packages/cdn_definitions/_impl/__init__.py:docstring of cdn_definitions._impl.origin_aliases:3: WARNING: Unexpected indentation.

So maybe it was not correct either before or after? However, the rendered docs seem OK to me, so no objections to this.