get_most_fish() causes Type error

[tms-hl]

If a field is occupied by a player, the method get_most_fish of Board throws:

TypeError: '<' not supported between instances of 'NoneType' and 'int'

Reason: The field list is sorted using get_fish(). get_fish returns None if the field is occupied.

[tms-hl]

Actually this seems easy to solve. Just added a pull request.