bom_csv_grouped_by_value_with_fp.py: fix compatibility with python < 3.9

From master branch

eeschema/plugins/python_scripts/bom_csv_grouped_by_value_with_fp.py

@@ -62,12 +62,11 @@ for group in grouped:
     # 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 += fromNetlistText( component.getRef() ) + ", "
+        if refs != "":
+            refs += ", "
+        refs += fromNetlistText( component.getRef() )
         c = component
 
-    # remove last, not needed trailing comma and whitespace
-    refs = refs.removesuffix(', ')
-
     # Fill in the component groups common data
     out.writerow([refs, len(group),
         fromNetlistText( c.getValue() ),