Add type hints, py.typed
It'd be nice to have type hints and a py.typed file to check this repository (and any code using this Python package) can use mypy or the like. This also gives better auto completion and what not.
I believe that #101 blocks this.
Py2 usage dropped below 2% of downloads, so we might do it. PR is welcome, if you prepare it I won't have any objections to include it.
Do you have any preference in terms of python 3.10 style for type hints (e.g. varname: dict(str, float | int) = ...) or earlier version style (e.g. varname: Dict[str, Union[float, int]] = ...)
I definitely prefer the Python 3.10 syntax for unions, but I think it'd be a bit premature to switch to it. I think 3.7+ would be ideal as they are the only supported versions.
From my side there's no preference as I never used type annotation. However, backward compatibility is important, so please keep to the most popular version (according to PyPi stats, 3.7 dominates the downloads).