Twined

Summary
Use the latest JSON metaschema and switch to HTTPS to access it.

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Contents ([100](https://github.com/octue/twined/pull/100))

Enhancements
- Use latest JSON metaschema `2020-12` and access it via HTTPS

Fixes
- Fix labels ref in manifest schema

Dependencies
- Upgrade to `jsonschema==4.4.0`

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Summary
Remove the credentials environment variable name from the children schema. The default `GOOGLE_APPLICATION_CREDENTIALS` is the only name now used in `octue-sdk-python`.

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Contents ([98](https://github.com/octue/twined/pull/98))

Enhancements
- Remove the credentials environment variable name from the children schema
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Summary
Simplify specification of datasets in twines and manifests.

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Contents ([95](https://github.com/octue/twined/pull/95))
**IMPORTANT:** There are 2 breaking changes.

Enhancements
- **BREAKING CHANGE:** Require "datasets" field to be a dictionary in `manifest.json` files
- **BREAKING CHANGE:** Require "datasets" field in the "manifest" strands of `twine.json` to be a dictionary
- Raise error if JSON objects contain duplicate keys

Fixes
- Use correct method to transform manifests during validation
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Because we ran out of free credit on Travis!

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Contents ([94](https://github.com/octue/twined/pull/94))

Operations
- Skip pull request description update if skip indicator in body

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Contents ([93](https://github.com/octue/twined/pull/93))

Operations
- Add and apply `pydocstyle` docstring linting to `pre-commit`
- Add and apply `pep8-naming` `flake8` extension to `pre-commit`

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