CircleCI Refactoring

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• update to 2.1
• use org context for environment variables, where possible
• use circleci for publish to npm / pip registries
• use orbs where possible
• use executors
• use slack orb

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see #415, #210

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To enable the context sections, we should have the following contexts with the following environment variables:

• org-public
  • PYPI_REPOSITORY=https://upload.pypi.org/legacy/
  • PYPI_USERNAME
  • PYPI_PASSWORD
  • NPM_REPOSITORY=registry.npmjs.org/
  • NPM_TOKEN
• org-publc-test
  • PYPI_REPOSITORY=https://test.pypi.org/legacy/
  • PYPI_USERNAME
  • PYPI_PASSWORD
  • NPM_REPOSITORY=registry.npmjs.org/
  • NPM_TOKEN