python-chess v0.6.0

New in this release:

• If there are comments in a game before the first move, these are now assigned
  to Game.comment instead of Game.starting_comment. Game.starting_comment
  is ignored from now on. Game.starts_variation() is no longer true.
  The first child node of a game can no longer have a starting comment.
  It is possible to have a game with Game.comment set, that is otherwise
  completely empty.
• Fix export of games with variations. Previously the moves were exported in
  an unusual (i.e. wrong) order.
• Install gmpy2 or gmpy if you want to use slightly faster binary
  operations.
• Ignore superfluous variation opening brackets in PGN files.
• Add GameNode.san().
• Remove sparse_pop_count(). Just use pop_count().
• Remove next_bit(). Now use bit_scan().