Xdg-base-dirs

This patch release no longer installs the `LICENSE` file into the `site-packages` directory.

This major release renames `xdg` to `xdg-base-dirs` due to an import collision with [PyXDG](https://pypi.org/project/pyxdg/). If you used `xdg` prior to the rename, update by changing the dependency name from `xdg` to `xdg-base-dirs` and the import from `xdg` to `xdg_base_dirs`. Python 3.10 or later is required by `xdg-base-dirs`.

This patch release moves to using [`pathlib.Path.home()`](https://docs.python.org/3/library/pathlib.html#pathlib.Path.home) to get the home directory.

This minor release implements version 0.8 of the XDG Base Directory Specification, released on 8th May 2021. Specifically, it adds a `xdg_state_home` function which returns a path to a directory suitable for storing application state, such as undo history or window layout.

This patch release fixes an issue with relative paths. Prior to this release, `xdg` would use paths defined by the relevant environment variables even if they were relative paths, which is not permitted by the specification:

> All paths set in these environment variables must be absolute. If an implementation encounters a relative path in any of these variables it should consider the path invalid and ignore it.

Now only absolute paths are accepted, and relative paths are ignored.

This patch release changes the build backend to `poetry-core` in order to make PEP 517 builds faster, and not require Poetry and all its dependencies be installed.