Invenio-pidrelations

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- Dependencies update
- Python 3 update
- Execution of RFC 21: removal of examples app

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- Model update by adding `relation_type` as primary key
- Dependencies update
- Reduce travis builds

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- Dependencies update
- API refactoring by making it more node-centric
- License change to MIT

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- Initial public release.

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- global: remove deprecated elasticsearch imports
- remove deprecated flask-babelex import
- CI: update test config
- installation: upgrade invenio-search
- tests: update run-tests script
- installation: update extras' keys
- installation: upgrade pytest invenio
- blueprints: fix naming (deprecation of dots usage)
- tests: update fixtures settings

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- Migrate CI to GitHub actions