Openfisca-country-template

* Tax and benefit system evolution.
* Impacted periods: from 01/01/2015.
* Impacted areas: `disposable_income`
* Details:
- Calculate `disposable_income` for the entire household
- Include taxes and benefits in `disposable income`
- Allow for triplets in households

* Crash fix
* Impacted areas: `first-time-setup.sh`
* Details:
- Trying to adapt file `.flake8` would crash setup script when file is not present
- This changeset fixes that problem

* Crash fix
* Impacted areas: `pyproject.toml`
* Details:
- Changed `openfisca-country_template` to `openfisca-country-template` by mistake
- This changeset fixes that problem

* Technical improvement.
* Details:
- Run tests in absolute isolation
- Uses `tox` to test builds in isolation

* Minor change.
* Impacted areas: `**/*`
* Details:
- Normalize `pyproject.toml`
- Update OpenFisca-Core to 43.0.0

* Minor change.
- Update OpenFisca-Core to 42.0.0