Resolve "Make compatible to h5py >= 3.0"

This MR updates some tests in order to be compatible to h5py >= 3.0. As of that version, h5py takes care of bytestring decoding, something that dantro had to do manually prior to h5py 3.0. This breaking change led to a test failing.

The solution is:

• Adjust the test: dantro no longer needs to do this for regular strings (we still need to do it for encoded strings that are part of an array!)
• Bump the minimum required h5py version to 3.1

Can this MR be accepted?

• Implementation ready no changes needed
• Tests added or adjusted
• Documentation extended or updated

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

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

Related issues

Closes #212 (closed)