Release v1.7.0

• Added: experimental support for text files
  as media files
• Added: dependency on pyarrow
• Added: audb.Repository.backend_registry
  that maps repository names like artifactory
  to corresponding backend classes,
  e.g. audbackend.backend.Artifactory
• Added: audb.Repository.register()
  to add an entry to audb.Repository.backend_registry
• Added: audb.Repository.create_backend_interface()
  returns a backend interface
  to access files in the repository
• Changed: improve speed
  of loading dependency table to the cache.
  E.g. for version 1.0.0 of the database musan
  loading time is reduced by 35%
• Changed: improve speed
  of downloading a database to the cache.
  E.g. for version 1.0.0 of the database musan
  loading time is reduced by 40%
  when using 8 threads
• Changed: depend on audbackend>=2.0.0
• Changed: dependency table dataframe
  returned by audb.Dependencies.__call__()
  now uses pyarrow based data types
• Changed: dependency table
  is now stored as a PARQUET file
  on the backend,
  instead as a CSV file
• Fixed: audb.versions()
  for non-existing repositories
• Fixed: documentation of audb.Repository.__eq__()