Improve package's typing

[bonjourmauko]

Hi there!

Here is what I did:

I used a function expecting an int as argument, with a str argument.

Here is what I expected to happen:

To get an error from make check-types.

Here is what actually happened:

Nothing.

Context:

Type-hinting is an inexpensive way of :

• Documenting the code
• Building the documentation
• Finding an correcting bugs —I've spotted quite some already
• Helping contributors/maintainers enforce contracts, with zero runtime impact to reusers
• Finally, they're a just part of the standard core of Python, like using a list or concatenating a string

TODO

• [x] #1054
• [ ] #1167
• [ ] 3. Improve openfisca_core.entities typing
• [ ] 4. Improve openfisca_core.holders typing
• [ ] 5. Improve openfisca_core.indexed_enums typing
• [ ] 6. Improve openfisca_core.parameters typing
• [ ] 7. Improve openfisca_core.periods typing
• [ ] 8. Improve openfisca_core.populations typing
• [ ] 9. Improve openfisca_core.projectors typing
• [ ] 10. Improve openfisca_core.reforms typing
• [ ] 11. Improve openfisca_core.simulation_builder typing
• [ ] 12. Improve openfisca_core.simulations typing
• [ ] 13. Improve openfisca_core.taxbenefitsystems typing
• [ ] 14. Improve openfisca_core.taxscales typing
• [ ] 15. Improve openfisca_core.tracers typing
• [ ] 16. Improve openfisca_core.variables typing
• [ ] 17. Improve openfisca_web_api.loader typing