Improve usability of Enum values

[Morendil]

Allow the use of Enum values in comparisons, working around our abuse of the Python module system for TaxBenefitSystem loading.

Tested by updating test_holders.py to import HousingOccupancyStatus instead of getting possible_values. (Also tested, outside of CI, against the YAML test in this branch of France.)

[sandcha]

Might be tested with this code calling openfisca-tunisia :

from openfisca_tunisia import CountryTaxBenefitSystem as TunisiaTaxBenefitSystem
from openfisca_core.simulation_builder import SimulationBuilder

from openfisca_tunisia.model.prelevements_obligatoires.cotisations_sociales import TypesRegimeSecuriteSociale

tax_benefit_system = TunisiaTaxBenefitSystem()

nb_individus = 3
simulation_builder = SimulationBuilder()
simulation = simulation_builder.build_default_simulation(tax_benefit_system, count=nb_individus)

period = '2019-01'
simulation.set_input('salaire_de_base', period, [1000, 1000, 1000])

regimes = [TypesRegimeSecuriteSociale.rtns, TypesRegimeSecuriteSociale.raci, TypesRegimeSecuriteSociale.salarie_cnrps]
simulation.set_input('regime_securite_sociale', period, regimes)

ugtt = simulation.calculate('ugtt', period)
assert ugtt[2] == -3, ugtt[2]

[Morendil]

@sandcha That looks like it would work, and OpenFisca Tunisia needs to be updated to Core 25 first.

[sandcha]

Thank you @Morendil!
It resolves the initial issue on openfisca-tunisia.

[sandcha]

Shorten comment or keep in PR only?

Suggested change

	# the values in the array, if non-empty, are of the right type.
	# "cls" generally comes from variables.possible_values (thus, from load_module)
	# while the array values may come from directly importing a module containing an Enum class.

[Morendil]

As you know I'm not a fan of long comments in code, even of comments in general. In this particular case I really feel that the somewhat long explanation is needed to make sense of the code that follows…

[sandcha]

👍