Wikidata values for catalog providers

[marfox]

This PR closes #418 . Please bear in mind that relevant refactoring suggestions from #419 are not yet addressed here: we have opened issues accordingly (#420 #421 #422) and will tackle them as soon as all the new validator features are implemented.

[marfox]

@e-dorigatti : friendly ping

[marfox]

Many thanks for the review, your feedback is always super constructive and much appreciated.

return values need to be made consistent with type annotations.

Fixed.

you may look into pre-commit to automatically run type checking, linting, formatting, etc. on commit

Very interesting suggestion! We currently have a similar mechanism in place on Travis CI, see this dotfile. However, it's a bit hacky and doesn't run mypy type hints check. From a first glimpse, it looks like pre-commit is a much better solution:

• it can cater for CI , see https://pre-commit.com/#usage-in-continuous-integration;
• no hack seems required to autofix PRs, see https://pre-commit.com/#pre-commitci-example;
• it's claimed to be faster than Travis, see https://pre-commit.com/#pre-commitci-example.