Mkdocs-merge

- Fixed bug of `dir_util.copy_tree` caused by setuptools moving to 70.2.0 (fixes [20](https://github.com/ovasquez/mkdocs-merge/issues/20)).
- Updated dependency on deprecated distutils package to use setuptools version.
- Updated project to use `pyproject.toml` instead of `setup.py` (package version now has to be manually kept in sync).

**Full Changelog**: https://github.com/ovasquez/mkdocs-merge/compare/0.8.0...0.9.0

Added support for section index pages [feature from MkDocs Material](https://squidfunk.github.io/mkdocs-material/setup/setting-up-navigation/#section-index-pages) (thanks to [Acerinth](https://github.com/Acerinth)).

- Updated YAML dependency to support new YAML features (15).
- Updated other dependencies to recent versions.
- Migrated build system to GitHub Actions.
- Migrated tests from nose to pytest.

**Breaking change:** added support for MkDocs 1.0 and dropped support for earlier versions.

- Fixed the merge process ignoring the `docs` property in the `mkdocs.yml` of the sites to merge.
- Removed support for Python 3.3 due to pip removing support for it.

Fixed errors in 0.4.0 when using MkDocs Merge via CLI, or in Windows.