Doorstop

- **BREAKING:** Dropped support for Python 3.6, 3.7, and 3.8.
- **BREAKING:** Removed `--no-body-levels` option to `doorstop publish`.
- Fixed overriding default attributes with `doorstop add`. ([tangoalx](https://github.com/doorstop-dev/doorstop/pull/544))
- Fixed encoding error with special characters on Windows. ([urbasus](https://github.com/doorstop-dev/doorstop/pull/526))
- Added support for publishing to the LaTeX format. ([neerdoc](https://github.com/doorstop-dev/doorstop/pull/545))
- Added support for storing items in Markdown with YAML front matter. ([ckolumbus](https://github.com/doorstop-dev/doorstop/pull/566))

- Upgrade PyYAML to 6.x to fix installation on newer versions of Python.

- Added support for UTF-8 symbols in references. ([stanislaw](https://github.com/doorstop-dev/doorstop/pull/485))
- Added custom attributes to published documents. ([chrta](https://github.com/doorstop-dev/doorstop/pull/476))
- Fixed handling of special characters when reordering items. ([asimon-1](https://github.com/doorstop-dev/doorstop/pull/521))

- Updated `markdown` to fix `AttributeError` in `doorstop-server`.

- Removed `mdx_outline` Markdown extension, which is incompatible with Python 3.9.

- Fixed tree builder to skip virtual environments.