Tweak CI jobs for minimum dependencies and reduce scipy minimum bound

The scipy change will allow a conda-forge deployment, where scipy 1.5.4 is not available currently.

Can this MR be accepted?

• Implementation ready
• Implementation working
  • minimum dependency jobs not running in this MR
  • ... unless title was changed to contain "Prepare release"
• Documentation extended or updated
  • Readme
  • GitLab Release MR template

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved

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