Openfisca-core

- Improve usability of Enum values:
- Details:
- Allow the use of Enum values in comparisons: instead of using `<Enum class>.possible_values` you can simply `import` the Enum class
- Accept Enum values via set_input (same result as the previous point)

Breaking changes

- Require clients to make explicit when input variables cover a range of dates, rather than allowing inputs to be derived from past or future values; also removes the `missing_value` base function

Migration notes

You might need to change your code if any of the following applies:
- you are a model author, and there are variables in your model that use a `base_function` attribute
- your model **or application** uses any non-numeric variables (boolean, date, string or Enum) used in, for which the period on which you define **inputs** does not match the period for which you are requesting **outputs**

Detailed instructions are provided in the [PR description](https://github.com/openfisca/openfisca-core/pull/813).

- Raise an error instead of silently ignoring the input when a user tries to set an input for a variable for several months (or several years), but the variable has no `set_input` declared.

- Apply `flake8-bugbear` recommendations and enforce same in continuous integration.

- Remove Python 2 compatibility code.

Breaking changes

- Improve cycle and spiral detection, giving consistent results more systematically

Migration notes

- Remove all optional parameters `max_nb_cycles`
- Avoid relying on cached values of a computation

For additional details, see the PR's [description](https://github.com/openfisca/openfisca-core/pull/817).