Migrate to pyproject.toml format

[thisisparker]

I'm feeling pretty persuaded by this recent Simon Willison blog post about using a single pyproject.toml file to define a Python package's metadata

[mixographer]

I don't really understand any of that blog post, but I do know that now it is super hard to install and update xword-dl. I don't think it's an issue with your project, I just think Python itself has become super-complicated.