Viur-core

- fix: access-decorator (1257)
- fix: Delete bones set to `None` from a Skeleton (1258)

- fix: `Skeleton.toDB()`s `is_add` determined wrong (1248)
- feat: Improve `CaptchaBone` (1243)

- feat: Extend `CONTRIBUTING.md` with Coding Conventions (1233)
- fix: `File`-module allows to upload into non-existing node (1235)
- fix: `MultipleConstraints` as intended (1228)
- fix: Improve `NumericBone.singleValueFromClient` (1245)
- fix: Inconsistency raises AssertionError (1237)
- fix: null-key always written into `viur-relations` (1238)
- refactor: `__build_app` function clean-up and make all modules accessible (1240)
- refactor: Improved `RelationalConsistency.PreventDeletion` validation (1244)
- refactor: Move datastore index retrieval to `SkelModule` (1239)

- doc: Fix RelationalBone docstring (1226)
- chore: Use `pyproject.toml` as new SSOT packaging system (1224)
- feat-fix: Wrap `descr` and `params.category` in `translate` object for auto translating (1223)

- feat: Add `EmailTransportAppengine` as default email transport class (1210)
- feat: Improve email attachments (1216)
- feat: Render `SelectBone` values in dict-style (1203)
- fix: `RecordBone.getReferencedBlobs` should collect references for all bones (1213)
- fix: `viur-core-migrate-config` should replace sendinblue and mailjet configs as well (1200)
- refactor: `email`-module/`EmailTransportMailjet` fixes (1212)

- fix: Changed `EmailTransportMailjet` mimetype detection to `puremagic` (1196)