Use reversed() instead of this weird inverted manual for loop stuff

[rpdelaney]

This is one of those "what was I thinking?" moments now that I've had some time to level up my python since I wrote this.

[rpdelaney]

Might require changes to python-chess.

https:/docs.python.org/3/library/functions.html#reversed

reversed(seq)

    Return a reverse iterator. seq must be an object which has 
a __reversed__() method or supports the sequence protocol 
(the __len__() method and the __getitem__() method with integer 
arguments starting at 0).

[rpdelaney]

Consider extending with a superclass also, in the event that python-chess can't be changed.

[rpdelaney]

niklasf/python-chess/issues/319 enabled use of reversed(node.mainline()): node.mainline() now returns an iterator of GameNodes, not Moves, and .mainline() supports the sequence protocol.