Openfisca-country-template

* Technical improvement.
* Impacted areas: tests/variables and parameters.
* Details:
- Add a new variable called parenting_allowance to show how group entities and single entities can be used together.
- This variable calls the household_income variable.

> Note: Version `3.12.1` wasn't unpublished by mistake. Please use version `3.12.2` or more recent.

> Note: Version `3.12.0` wasn't unpublished by mistake. Please use version `3.12.2` or more recent.

* Technical change
* Details:
- Declare package compatible with OpenFisca-Core v35 that updates `numpy` dependency.

* Tax and benefit system evolution.
* Impacted periods: all.
* Impacted areas: `reforms/add_new_tax.py`.
* Details:
- Add a new reform example creating a variable (there was none prior).
- The example is a new tax that adds a fixed 100.0 of the country's currency to the actual income tax.

* Technical improvement.
* Impacted areas: `**/*`.
* Details:
- Remove wildcard imports as they're considered and [anti-pattern](https://docs.quantifiedcode.com/python-anti-patterns/maintainability/from_module_import_all_used.html).