Revert OpenAPI version number to 3.0.0

[MattiSG]

This was bumped in 8be2ec4e7b598c70111a30458ebe6d8e40d96abc (#989). We do not actually abide by 3.1.0, as can be seen in the use of example instead of examples, support for which was dropped. v3.1.0 is indeed a breaking change, but the OpenAPI technical committee decided not to follow SemVer.

Support for OpenAPI v3.1 is not offered by the current Legislation Explorer version and upgrading is too costly (see https://github.com/openfisca/legislation-explorer/issues/236).

Technical changes

• Abide by OpenAPI v3.0.0 instead of v3.1.0
  • Drop support for propertyNames in Values definition

[coveralls]

coverage: 74.48%. remained the same when pulling 1c655501c78763820b8952df3ff92c9f03d80c5d on openapi3 into d5d287d1ebcf634400f3333712ee6cb5fb78a08a on master.