Drop Python 3.9 support, create beetcamp module, small cleanup fixes

Removed

• Drop support for Python 3.9 (EOL since October 2025).

Fixed

• Fix truncating comments that contain unicode characters.

• (#83) Fix beets installation in CI.

• catalognum:

  • Handle formerly known as label names, where the former label name is part of the
    catalogue number.

• album:

  • Handle edge case when parsed catalogue number does not have a space, say ABC123
    but it appears in the album name with a space, ABC 123.
  • Fix several edge cases when artist was wrongly removed from the album name.

• albumartist:

  • Remove remixer lists like (Remixes by ... or w/ ... remixes

• artist:

  • In compilations where no artists are found, try splitting titles with a dash -
    without surrounding spaces.

• cleanup:

  • Fix a couple of edge cases cleaning up digital-only artefacts.
  • Remove PREMIERE from the start of album and track names.
  • Find catalogue numbers with a shared prefix in track titles and remove them.

Added

• cover_art_url: Add cover_art_url field to albums and singletons which is used by
  beets>=2.4.0 for artwork resolution.
• Create beetcamp module for the core functionality to speed up the tool when it is used
  as CLI.

Updated

• genre: Update musicbrainz genre list