Openfisca-core

Minor Change without any impact for country package developers and users:
- Make code more Python3-like by backporting unicode litterals.
- With this backport, all strings are by default unicodes.
- The `u` prefix for strings should *not* be used anymore.
- Each new module must start by `from __future__ import unicode_literals` for the backport to be effective.

* Send reference of the country-package and its version to the tracker so it will appear in the tracking statistics.

* Change the way metadata are declared for Parameter.

Before:
YAML
description: Age of retirement
reference: https://wikipedia.org/wiki/retirement
unit: year
values: (...)


After:
YAML
description: Age of retirement
metadata:
reference: https://wikipedia.org/wiki/retirement
unit: year
values: (...)


_Setting `unit` and `reference` out of `metadata` is considered deprecated, but still works for backward compatibility._

* Allow legislation coders to define their own medatada

* Expose in the Python API
- Parameters metadata:
- e.g. `parameters.taxes.rate.metadata['unit']`
- Parameter value metadata:
- e.g. `parameters.taxes.rate.values_list[0].metadata['unit']`
- Parameter node description and metadata:
- e.g. `parameters.taxes.metadata['reference']`, `parameters.taxes.description`
- Note: Parameter descriptions (e.g. `parameters.taxes.rate.description`) were already exposed

* Introduce `TaxBenefitSystem.replace_variable`
- Unlike `update_variable`, this method does _not_ keep any of the replaced variable in the new one.
- See [reference documentation](https://openfisca.org/doc/openfisca-python-api/tax-benefit-system.html#openfisca_core.taxbenefitsystems.TaxBenefitSystem.replace_variable).

* Fix installation on Windows with Python 3.7
- Require `psutil` version `5.4.6`, as `5.4.2` is incompatible with that environment.

* In the error message sent to a user trying to set a variable without specifying for which period, add an example for a variable defined for `ETERNITY`.