use relative imports to avoid circular import issues

Author: edwardchalstrey1

src/pygambit/catalog/catalog.py

@@ -4,7 +4,7 @@
 
 import yaml
 
-from pygambit.gambit import Game, read_efg, read_nfg
+from ..gambit import Game, read_efg, read_nfg
 
 _GAMEFILES_DIR = Path(__file__).parent.parent.parent.parent / "contrib/games"
 

src/pygambit/catalog/coded_games.py

@@ -1,5 +1,4 @@
-from pygambit.gambit import Game
-
+from ..gambit import Game
 from .catalog import CatalogGame