docs: add typing to commons

[bonjourmauko]

Fixes #1230
Follows discussion on #1033

New Features

• Introduce openfisca_core.types

Documentation

• Complete typing of the commons module

[MattiSG]

After a video discussion with @maukoquiroga and a bit of research (see also #1033 (comment)), I understand that adding type hints:

• Will have no functional impact on OpenFisca.
• Will not force contributors to add their own type hints.
• Will not force country packages to add their own type hints.
• Will improve generated documentation.
• Will improve readability of the code base.
• Will improve testability of the code base.
• Opens up the way for future optimisations: if the whole codebase gets type-annotated, it will be easier to compile, to statically analyse, and to optimise for performance.

In order to minimise install performance side-effects and burden, I requested that additional dependencies are kept to a minimum, which has been done today.

Under these conditions, I believe this changeset adds value and does not warrant a preliminary community agreement, as it should be considered mainly as documentation improvement.
I suggest type hints to be encouraged in future PRs, but not required. Once production and contribution feedback has been obtained, we should consider opening an RFC on making type hints mandatory for future contributions along with strict CI checks.

[MattiSG]

I see that my approving review has been dismissed. Please let me know if there is anything I can do in that regard @maukoquiroga 🙂

[bonjourmauko]

@MattiSG As soon as circleci passes, this should be LGTM :)

[MattiSG]

New changes reviewed & approved 😉

My previous review was auto-dismissed (cf. #990 (comment)).

[bonjourmauko]

This stills needs your review @benjello 😃

[bonjourmauko]

There was a weird issue in circleci regarding the version check, however I cannot understand the cause. All looks good to me.

[bonjourmauko]

Thanks @benjello !