Log Message:
Changes 1.38:
* Note: New release tarballs are now hosted on github:
  https://github.com/quodlibet/mutagen/releases
* ID3:
  * Add iTunes grouping frame `id3.GRP1`
  * Fix exposing text frames where the text can't be encoded with the
    reported encoding due to merging of frames
* OGG: Fix wrong StreamInfo.length (small negative value) for all
  ogg based formats in rare cases.