`mergedeep` seems to be no longer maintained

It looks like the [mergedeep](https://pypi.org/project/mergedeep/) project is no longer [maintained](https://github.com/clarketm/mergedeep/issues/20).  Maybe it is time to [replace it](https://github.com/mkdocs/mkdocs/blob/2862536793b3c67d9d83c33e0dd6d50a791928f8/mkdocs/utils/yaml.py#L9) by something else.