python-chess v1.0.0

Changes:

* Now requires Python 3.7+.
* `chess.engine` will now cut off illegal principal variations at the first
  illegal move instead of discarding them entirely.
* `chess.engine.EngineProtocol` renamed to `chess.engine.Protocol`.
* `chess.engine.Option` is no longer a named tuple.
* Renamed `chess.gaviota` internals.
* Relaxed type annotations of `chess.pgn.GameNode.variation()` and related
  methods.
* Changed default colors of `chess.svg.Arrow` and
  `chess.pgn.GameNode.arrows()`. These can be overriden with the new
  `chess.svg.board(..., colors)` feature.
* Documentation improvements. Will now show type aliases like `chess.Square`
  instead of `int`.

Bugfixes:

* Fix insufficient material with same-color bishops on both sides.
* Clarify that `chess.Board.can_claim_draw()` and related methods refer to
  claims by the player to move. Three-fold repetition could already be claimed
  before making the final repeating move. `chess.Board.can_claim_fifty_moves()`
  now also allows a claim before the final repeating move. The previous
  behavior is `chess.Board.is_fifty_moves()`.
* Fix parsing of green arrows/circles in `chess.pgn.GameNode.arrows()`.
* Fix overloaded type signature of `chess.engine.Protocol.engine()`.

New features:

* Added `chess.parse_square()`, to be used instead of
  `chess.SQUARE_NAMES.index()`.
* Added `chess.Board.apply_mirror()`.
* Added `chess.svg.board(..., colors)`, to allow overriding the default theme.