Add type hints

[getzze]

This is a huge PR, but hopefully it is not too hard to review.

To type check the code I had to setup mypy and I found it was simpler to also add a pyproject.toml file and bump the minimal python version to 3.8 (3.7 is not supported anymore anyway). I also set up a ruff workflow in the pyproject.toml file, I find it very useful to lint and format the code.

All tests are passing, mypy and ruff give no errors.

The problem to review this PR is that I modified the python code sometimes, to improve type checking. Maybe I can add comments where the code was modified to make it easier to review. Some examples of things that I modified:

• make no_default an Enum sentinel: using the string "__no_default__" do not work anymore
• same for no_pad.
• split def some_func(*args, ...) in def some_func(func, /, *args, ...) when args needed to be a function followed by its arguments.
• use an explicit list of keyword arguments instead of **kwargs in function definitions to be able to type check the arguments.
• add missing returns
• transform assert to if blocks.

Also, I added a dependency to typing_extensions, that is a backport of typing improvements to older python versions. It can be removed, losing a bit of type checking for functoolz.excepts and functoolz.juxt.

Related PR: #516 #502 Issue: #496

[eriknw]

wow, nice! Thanks @getzze! I'm in the process of moving so it may take me some time to properly review this, but know that this is appreciated and likely to go in :)