Release v1.15.0

• Added: support for Python 3.15
• Changed: the repository argument in audb.publish() is now optional,
  when previous_version is not None.
  The database is then published in the same repository as its previous version
• Changed: lock cache with filelock.FileLock instead of filelock.SoftFileLock.
  This has the advantage that the cache can no longer be blocked by leftover lock files.
  The downside is that we no longer support shared caches across different platforms
• Removed: support for Python 3.10
• Fixed: missing space character in verbose message of audb.publish()