Add test environments for Python 3.10

This MR adds Python 3.10 environments in the CI to test against this new Python version (to be released 3 days from now).

Can this MR be accepted?

• Implementation ready
• 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

Related issues