BOM Generation: Unifying reference concatenation using more pythonic string join to avoid trailing comma.

eeschema/python_scripts/bom_csv_grouped_extra.py

@@ -94,16 +94,16 @@ grouped = net.groupComponents()
 index = 1
 for group in grouped:
     refs = ""
+    refs_l = []
 
     # Add the reference of every component in the group and keep a reference
     # to the component so that the other data can be filled in once per group
     for component in group:
-        refs += component.getRef() + ", "
+        refs_l.append( component.getRef() )
         c = component
-
-    # Remove trailing comma
-    refs = refs[:-2]
-
+    
+    refs = ", ".join(refs_l)
+    
     # Fill in the component groups common data
     row = []
     row.append( index )