feature: add type hints for recipes (#496)

[Jiehong]

I decided to give this a try with the smallest file I've found, so that we agree on the direction this goes.

Some notes:

• I did not use automatic generation of types: I wrote them by hand based on my understanding;
• I used 'A' and 'B' for typevars, instead of 'T' or '_T', because I find them more explicit (I dislike the C++ notation);
• I checked, and those types should be available once published with nothing to do (to check).

I was thinking of setting up mypy in the CI/CD to check the type validity, what do you think?

[eriknw]

I like where this is going; thanks @Jiehong!

I like how you used A and B. I'm all for setting up mypy check in CI.

Let's keep going!

[Jiehong]

I've updated the PR with mypy running on recipes.py only. I had to ignore some issues in some lines by adding some comments to make it pass.

Mypy also helped choosing between Iterable and Sequence fairly easily :D

[Jiehong]

pypy seems to have issues with mypy installation, somehow.

[Jiehong]

@eriknw : this PR is now ready for review:

• mypy is setup (but only when not using pypy, as the installation fails);
• mypy reports no errors;
• recipes.py is correctly typed.

If you're fine with this PR, then it can be merged.

I'll make new PRs for another file only after that. I want to avoid make 1 huge PR to add types everywhere at once.