Hi! Thanks for the PR, and acknowledging that I've seen this.

My major concern is that this package is used by several other projects, at least one of which is still on python 2.7 (I know, I know), and it would be nice to keep it supported with the latest versions. Worse case scenario we can drop it, but let me fiddle with it for the next few days and see if I can have my cake and eat it too.

Less seriously, I don't love the .pyi files (mostly because I don't use PyCharm, and could very easily forget to update the other file), though I understand the desire for type hinting. I may .gitignore them for now (depending on the outcome of the above).

My thought would be that clients still on old versions of Python might have to restrict their version of this package to something like <1.5.0. The .pyi files are for anything that can use them: mypy or IDEs like VS Code and PyCharm. Other than requiring them to be syntactically valid, Python ignores them. As for making sure that the .py and .pyi files don't diverge, we can add mypy to the CI script. Ordinarily, I'd put the annotations inline with the code for readability; however, annotations weren't introduced until Python 3.5 and some of the features I used didn't appear until Python 3.9 or so. Thanks for considering.

Clarification: inline annotations need to be syntactically valid Python (versions 3.5 and above depending on type annotation features). Annotations in .pyi files are only read by and relevant to typing tools like mypy or IDEs.