Xdg-base-dirs

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.

This major release refactors the API to provide functions rather than variables. This means `xdg` will now respect changes to environment variables made after it is imported. The previous variable based API is maintained for backward compatibility, but is no longer documented.

This patch release adds a `py.typed` file to mark `xdg` as PEP 561 compliant.

With this major release, paths are no longer returned as strings but as [`pathlib.Path` objects](https://docs.python.org/3/library/pathlib.html#pathlib.Path), which are the new de facto standard for specifying file system paths in Python.

If your code expects strings rather than `pathlib.Path` objects, wrap the variable in a call to `os.fspath`, e.g.
python
import os
import xdg

config_home = os.fspath(xdg.XDG_CONFIG_HOME)

This patch release changes the test setup to allow easier distribution packaging. The test suite can now be run with `python setup.py test`.

The encoding of `README.rst` is now specified when it is opened in `setup.py`, to resolve an issue encountered when installing `xdg` with [Nix](https://nixos.org/nix/).