Unpinned version to fix bug on macOS big sur

@kel85uk 5 min review

Explanation of changes

Unpins pyqt5 version

Motivation of changes

Current pinned version does no work on macOS Big Sur (latest stable macOS). (https://stackoverflow.com/q/64818879)

This change should still cover the previous issues.

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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 has 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.