direct_api: Joint.connected_to is a set(), not singleton

Author: ezrec

src/build123d/direct_api.py

@@ -8011,7 +8011,7 @@ class Joint(ABC):
     def __init__(self, label: str, parent: Union[Solid, Compound]):
         self.label = label
         self.parent = parent
-        self.connected_to: Joint = None
+        self.connected_to: Joint = set()
 
     def connect_to(self, other: Joint, *args, **kwargs):  # pragma: no cover
         """Connect Joint self by repositioning other"""
@@ -8027,7 +8027,7 @@ def connect_to(self, other: Joint, *args, **kwargs):  # pragma: no cover
 
         other.parent.locate(self.parent.location * relative_location)
 
-        self.connected_to = other
+        self.connected_to.add(other)
 
     @abstractmethod
     def relative_to(self, other: Joint, *args, **kwargs) -> Location: