Let load_yml and write_yml use os.path.expanduser

This MR lets load_yml and write_yml use os.path.expanduser on the given path argument.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted (none necessary)
• 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
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @jeremiastraub

Related issues

Came up in !170 (merged)