Release 0.5.0

N.B. Let's hold off from releasing until Monday, due to the industry wisdom of never to deploy on Fridays. Staging the release already, but dated the changelog entry to 2021-10-25.

Explanation of changes

"Expanded feature sets hardware compilation backends". The most major changes are to the hardware backends and experiment flow is still not complete, since some key aspects are not yet fully there.

Updated the changelog.

Motivation of changes

The release is v0.5.0 since it contains breaking changes.

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Merge checklist

See also merge request guidelines

• Merge request has been reviewed and approved by a project maintainer.
• Merge request contains a clear description of the proposed changes and the issue it addresses.
• Merge request made onto appropriate branch (develop for most MRs).
• New code is fully tested.
• New code is documented and docstrings use numpydoc format.
• CHANGELOG.rst and AUTHORS.rst have been updated (when applicable).
• CI pipelines pass
  • black code-formatting passes (gitlab-ci),
  • test suite passes (gitlab-ci),
  • no degradation in code-coverage (codacy),
  • no (serious) new pylint code quality issues introduced (codacy),
  • documentation builds successfully (CI and readthedocs),
  • windows tests pass (manually triggered by maintainers before merging).

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For reference, the issues workflow is described in the contribution guidelines.